Data somehow transferred from Epic Games to Steam. Not complaining, just curious how this happened.

Gravity Circuit is currently free, so I nabbed it on Steam. I already had it when it was free on Epic a while back. For some reason, upon launching it on Steam, it immediately granted me a bunch of achievements that I would have unlocked had my initial playthrough been on Steam and my save file was there. I'm not complaining, just... confused on how this happened.