Every now and again I get paired with someone a few 100 points lower despite my filter being set.
This happens very rarely for me.

The other day, I noticed that it happened after I had just played a game and clicked on the
"New Opponent" button from within the game window.

I almost never do this.
I almost always go back to the main screen and click on the "Create A Game" button.

I haven't tested this theory yet, but it sounds like it might make sense.

Could it be this simple?

Does anyone have any feedback that can confirm/deny this?

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(I also set my phone to the same ratings filter and that seems to have helped too.)

Perhaps a game with a different time control was adopted.
if after that you just clicked "find another opponent"-time control remains from the past game

I think the "New Opponent" button negates the ratings filter in some instances.

If it was happening all of the time, I assume that this would have been discovered already.

But it's possible that some people with certain setups experience this 'glitch' when they click on "New Opponent" after a game, instead of going back to the main screen and selecting, "Create A Game".