Here is a detailed explaination of why from the Internet
http://tigertank.com/aslcrossroads/tactical/arty.htm:
American Artillery Practices:
Americans used the British system, but with a very significant innovation. They pre-computed the firing data for a HUGE number of variations of wind/temperature, barrel wear, elevation differentials, etc. Then for each possible variation, they created a separate calibrated tape measure. Along the tape was printed the gun laying information instead of distance marks. When a firing mission came in, the plotting officer would simply go to a filing cabinet containing the hundreds (thousands?) of these tapes and pull out the correct one for the current meteorological and situational factors. Then the tape would be laid out between the two grid points on the map (the battery's and the target's) and the firing data
would be read from the printing on the tape. Apparently there were some other fudges that got thrown in to make the firing even more accurate.
Net result was that there were about three minutes elapsed time from the initial fire support call until shells were making the enemy duck. And the firing was almost as accurate as the spotted German fires. Ergo, very responsive explosions exactly where they are wanted.
Again, a drawback to the American system is that it requires very accurate and detailed maps (say showing individual farm buildings for instance) which must be plentifully supplied to troops at all levels. However, given the availability of such maps then American artillery could be hellacious.
I might guess that temporary lack of such maps may be a reason why certain obvious movements were tardy during the pursuit across France. How would you feel about moving into an area where your artillery could not fire (because the forward troops as well as the artillery had no maps with appropriate grid marks)?
The tape measure system was not the only innovation of the Americans, as there were several others that followed directly from the simplicity of the tape usage.
Since the grid system was so easy to use for calling in fires, it was standard doctrine to train all officers in it (and many enlisted men as well?). In fact the technique was so easy,
that an otherwise ignorant enlisted man could be readily walked through the procedure over radio (and was on more than one occasion) when all his officers had fallen.
Another trick of the Americans, as Jim O'Neil has recently posted in detail, was the Time on Target mission (TOT). With this one, every battery in range was told the grid coordinates of the target and time when all shells were to initially land at the target. Each battery did its normal firing computation and then calculated the time to "pull the lanyards" by backing off the time-of-flight from the target time. TOT was particularly nasty because the initial shell from every gun landed virtually simultaneously before any defender could
take cover. It took too much effort for the Germans to care much for such a technique, and the British were not accurate enough to make the technique particularly useful. Very nasty and only Americans could pull it off (Jim claiming it required as little as 10 or 20 minuts reparation).
Another innovation of the Americans was their ability to obtain accurate fires extremely quickly from a LARGE number of firing batteries. Because of the simplicity and elegance of the tape system, almost any battery in range could fire on any target in any direction. All they had to do was get a request from another firing HQ or even just listen in on other battalion radio nets ("Hey, Red Bravo Two, we have a situation at grid coordinates such and so").
This system was formalized by having a fire mission request being kicked "upstairs" if warranted for a suitably attractive target. The firing artillery battalion might contact the
division which then might also request support from corps. Ostensibly, the inclusion of the division support added an additional three minutes to the fire mission, and including corps assets added three minutes yet again. There apparently was one case in Italy of a piper cub pilot (an artillery spotter) calling in no less than five corps level missions in one hour (this extremity of fire concentration was of course EXTREMELY uncommon, but certainly not unheard of).
Such relatively spontaneous massing of fires was absolutely not true of the German system which required a careful pre-plotting by surveyors to figure out where things really were on the map. In some sense, all American batteries wind up in general support (can fire for anybody). Consequently a given fire request may pick up extra "idle" batteries to thicken the fires. And during emergencies, any battery in range could leap into the fray to save a Yank ground pounder's tail.
Beyoond the above "standard" organizational doctrine, apparently Americans were quite capable of concentrating fire support on as large a scale as needed. I'll offer an example from the German counter-attack at Mortain in August of 1944 (from Saving the Breakout, Alwyn Fetherstone, 1993). Three American infantry companies were trapped by the Germans on top of a hill overlooking the valley that Mortain lies within (this was a bottle neck that a major part of the German attack had to pass through, if it was going to cut off Patton's breakout). The American infantry held out for something like two days against the better part of a panzer/panzer grenadier division that desperately wanted the lousy Yanks off of the hill. The only problem seems to have been that some twelve and a half battalions of Uncle Sam's artillery could be called on in the instant by the infantry, anywhere on the highly visible countryside for miles around. This not only prevented all daylight movement by the German attack, but completely thwarted any attack on the infantry itself, even at night. To imagine the effect of being a German attacking up that hill, think of being on a football field with some fifty to one hundred 20-odd pound TNT explosions going
off around you EVERY second (some two hundred guns each firing every 3 to say 8 seconds). Another way to think of it is to say that, in some sense, you might expect to have a shell land within touching distance of you every 15 seconds or so. Yep, I don't think the US needs to bow to anybody when it comes to an ability to deliver impromptu
concentrated fires. :-< :-< [dead Jerry's] --- [attempt at sick humor]
BTW as a side note, no artillery gun anywhere (in the US Army at any rate) ever fired more than about 800 rounds in any day (Trevor Dupuy, Search for Historical Records of High Rate Artillery Fire in Combat Situations, 1978). This was the extreme high, and a more typical high for any given battery is likely to be on the order of several rounds per gun per day. Apparently logistical limits more than anything tended to prevent firing a larger number of missions.
No doubt more than one German officer assumed he'd have at least the first 15 or 20 minutes of his surprise attack free of defensive artillery fire. And when the artillery did start to come in, he'd expect to be warned by the initial spotting rounds. Instead he found he was under immediate fire placed directly on his men while many were still crossing the start line. I'm sure it appeared to more than one German that the Americans must have known when and where such attacks were coming. No wonder some Germans were impressed with American artillery.