From: Montreal, Canada
I believe it has to do with the supply level such that the rate at which units regain action points is affected by the supply level. The higher the supply, the faster and more likely units will regain their full level of action points. I'm not sure if terrain plays a role either, but maybe someone with more knowledge of this, could weigh in as well.
Did you check the manual? I should have checked it too myself before replying so I will do so now.
“Excellence is never an accident. It is always the result of high intention, sincere effort, and intelligent execution; it represents the wise choice of many alternatives - choice, not chance, determines your destiny.”