Yes that is probably what it was!

I'll remember that even two+ days after the initial upload, there may still be reencoding. I thought this was only few hours after the creator uploads.

So the smaller filesize also is not necessarily a sign of lower quality. In this case it's because it gets a quick first encoding like zip with fast compression, and then it gets reencoded presumably by something much more CPU intensive.

That said from 500mb to 400mb is a bit dodgy but what do I know maybe VP9 is that good.