Joel wrote this...
ORIGINAL: Joel Billings
The AI is as smart as it can be at Normal, but it will not use certain grand strategies until it is give some of the benefits of the higher difficulty levels. If you are an experienced strategy gamer, you will ulitmately need to give the AI benefits to keep the game competitive. Some games hide the benefits better than others, but I have never played a computer game that allows you to play all sides that did not require help. We allow you to see the various benefits we give the AI in the different play levels, so you can change those that you don't like. We strongly suggest that once you become familiar with the game, you give the computer some supply and transport help. Although the AI does a lot in GGWaW, it just isn't close to being as good as a good human opponent. If you give it a few advantages, it will give you a good fight.
Joel gives some tantalizing clues like...
"will not use certain grand strategies unless at higher difficulties"
or
"Some games hide the benefits better than others"
Maybe he and others want to keep this secret lest it ruin the fun or other reason, and that's understandable. Others may want to know exactly what they are up against so they can balance the AI as they see fit versus their skill level.
GGWaW does do a better job than most of allowing one to modify AI characteristics but if its true that Normal = -1 disadvantage for the AI, I was wondering about other unpublished things. For instance, most know that "Fog of War" does not apply to the AI. Are there any others?