Well, GMAC recalculates percentiles every few years (usually every 18 months based on my experience), and yes, they go down. My 750 used to be 99th, and now it is 98th. It has to do with record keeping and how their system works. Technically, they should provide your score as a snapshot at that time instead of applying to the current scoring curve. E.g. if you were the fastest, you won a gold medal in Olympics, but if 4 years later someone broke your record, you are no longer the fastest but that does not mean they dispute you winning the Olympics.
It does hurt the most when admissions consultants recommend having an 80th percentile on your sections and having a 78th is not helpful from the optical perspective, at least. The Admissions Teams know that however, and they realize that you have taken your test a while ago.
Nothing that can be done from that perspective. The only part that should you make you feel safer, is that inflation of the overall score (e.g. 700, 750) is much slower than that of the individual sections. It seems people are much more likely to score high on the Q or V sections but still not quite on both of those.
Anyway... I will bring it up to the GMAC on my next call with them but I would not expect a fast resolution.