Happens all the time. Unfortunately AA.com, unline united.com, doesn't show how many seats are available for each inventory bucket, otherwise you can see the fluctuations yourself. Add to this the fact that thousands of fares are added/changed/removed every day, then you get the seemingly psychotic behavior you've witnessed. And it's all due to yield management (squeezing as much as possible from every seat by holding just the right number available for last-minute high-fare passengers) and meeting competitive offers/specials/new fares/etc.