Not sure if you meant this to imply that they pull from the same bucket, but they don't. There actually doesn't seem to be much of a correlation between U inventory (milesaver business award) and C inventory (upgrade).

A quick ExpertFlyer search could probably find lots of flights with C inventory but no U inventory. The reverse does happen, but not nearly as common in my experience.

It's as if AA is saying 'ok fine... we'll let you use your stupid upgrades, but you need to buy a ticket first and then we'll think about letting you move up front. But go ahead and keep those miles banked, because we'd rather let seats go out empty than let you use your miles"