I don't think it is necessarily fair or accurate to just "compare the spread between the best and the worst".

AAdvantage is by far the biggest program, and more significantly, they probably give have more miles awarded for activities other than flying than any of the other airlines. What does that mean? Since they're giving away so many miles that weren't earned on flights they are going to need to assign more seats for free travel just to have the same level of relative availability as another airline.