I mean, I can sort of understand why... the other four games are games I could imagine people preordering and buying day/week one etc. and were all big sellers (Well, Shadow of the Colossus wasn't a huge seller but I'm sure it did more than enough), whereas Sonic Forces always felt like one of those "wait for a sale" kind of games, and it looks like all the waiting paid off for people, since they eventually got it for free, myself included! :P
If VGChartz software tracking was anywhere close to accurate, it also sold a respectable 560k physical on PS4 in the first 3 months, probably around 900k-1m with digital included:
Not bad at all. I'm not too surprised though, even though Forces didn't get great reviews, 3D Sonic always was pretty popular. Unleashed and Generations both sold over 5m across all platforms I'm pretty sure.