I would guess (but don't know for sure) that in the old days of S&H green stamps, the catalog of premiums was revised on a regular basis, and the number of stamps needed for a particular item likely to change at least occasionally. But this is just a guess. Over time, that would probably have had to happen - since the number of stamps correlated to dollars spent.

In the case of flyer miles, the inflationary factor would be automatically compensated since the premium is for miles flown rather than dollars spent.

Interesting topic, which by the way, is not AA limited, so please don't move it!