Mouse speed/sensitivity changes when moving mouse at constant speed (Player Character Yaw)

Sometimes I will move the mouse and the sensitivity will change. Its quite difficult to notice visually but you can really feel the sluggishness and impreciseness.

I can prove and reproduce that the sensitivity changes and is inconsistent with a few tests.

First in the video I line up my mouse with a spot on the mountain and have my mouse pressed up against a metal bar so it cannot move anymore to the left. Then I make small slow movements right and left to prove my mouse always lands in the same spot. With a consistent sensitivity where I do not pick up my mouse and no acceleration is enabled, the same spot on my mousemat should always point my camera in the same direction so this is what im looking for. I then make longer and faster movements (but as linear as possible) and the camera begins to be offset and if you look closely there are times it moves faster or slower when I am moving at the same speed. Later at the end of the video I move to a spot and simply move my mouse left and right between 2 noted locations on my mousemat and watch as it gradually offsets my view. This does not appear in any other games I play.