Zone Deliveries to SHQ

[SeattleKCD]

I scanned through 10 pages or so and could not find a match to what I was looking for.

The amount of certain items that is delivered from the zone to SHQ seems to always be less than what remained after zone consumption. Food is a the item I see effected by this the most. My question is what determines this?

I have included a picture of an example, but the numbers were;
the zone produced 883 food, consumed 214 food, 450 food went to SHQ. Why was the amount sent to SHQ not 669 food?
Scrolling over the Out amount it says 450 was what could be delivered optimally, so it did not seem like a logistics issue.

[btonasse]

The math in your example doesnt add up exactly, but I've noticed the zone will try to keep 2 turns worth of consumption in stock