Right, score inflation alongside grade inflation. The administrators of the GMAT have an answer to that: https://www.gmac.com/why-gmac/gmac-news/gmnews/2017/july-201...

It says that it doesn't explain the "conundrum" of increased higher-end scores. But it definitely addresses that it's very unlikely the test has gotten easier, since average scores have remained stable, as they have for the PSAT and ACT.