Na.op.gg

op.gg uses an estimate. It isn't accurate.

Lets say you play against gold 3 players. It will say you have gold 3 MMR.

Now for a few games, you play against multiple Gold 4 duos. When you duo, it adds a bonus to the MMR it matches you with. So the Gold 4 duos will have Gold 3 MMR for matchmaking purposes to make up for the communication advantage they have.

You check OP.GG, and all it sees is Gold 4s in your game, its gonna say you have Gold 4 MMR, when in reality you're in Gold 3 MMR matches.