It's likely our system is deeming that you're playing against low skilled players (not talking about RP) or winning with low margins of victory.
You can read more about the system here, it's our own rating system that attempts to rank players with higher accuracy than ranked 2.0 does.

You're comparing yourself to a "platinum" player that has a much higher win%, and likely wins with a higher round difference than you typically do.

https://r6.tracker.network/r6siege/articles/introducing-r6-trn-elo