While waht Miles has said is correct on the surface, it is somewhat incorrect in practice. While Y,B and M fares are part of the "permanent" fare structure, there are in fact different Y, B and M fares for different seasons. Unfortunately Delta has not yet loaded "new" fares into the system yet, so the November fares will change (I have about 99% certainty of this). Therefore the fares shown for November are showing up the same as September (i.e shoulder or low season fares are currently showing up the exact same as high season fares).

The reasons for this are many, but in about a month time November fares will be lower.

However it is true that AA will allow a much wider range of fares to be upgraded, you have to actually have the upgrades available to get them.