It's a bit of an oxymoron, but what it means is that the game either misses reports or is trying to "smooth" them, resulting in the mouse movement being a lot longer than it should be. And it's not stable, the actual movmement is all over the place.
So it's the opposite of acceleration where the movement is shorter.
Using a high polling rate over 1000 Hz is not recommended as a lot of games struggle with this. Most handle 1000 Hz, but some (like this one) has smoothing enabled by default resulting in this negative acceleration.