Forgive me if this is wrong, but I’m pretty sure that you gain more Elo for the first few games you play on your Chess.com account.
I just found a chess.com article: https://www.chess.com/article/view/chess-ratings---how-they-work
it talks about the Glicko system and goes into depth about what you’re talking about.
(also, the fact that chess.com thought intermediate is 400 is quite strange. Usually on chess.com, intermediate is 1200, as you specified.)
Hey,
I regularly play on Lichess (~2000 Elo) and wanted to try out chess.com. I am fairly confident I clicked “intermediate” but my starting Elo is 400. I’m now up to 1200 on Chess.com but it’s been very boring and probably not too fun for my opponents. I’m only gaining <20 Elo per win now. What happened?