Yeah, it's the smoothing function that many devs leave on because it is on by default. In most cases it only affects 1000Hz, but in some cases also 500Hz and even 250Hz.
I haven't invested any effort into deciphering exactly what it does, but what I could find in the documentation is this:
And tick time is described as
Which might also explain why some UE4 games have the sensitivity directly tied to the FPS.
It's a horrible function and a mystery to me why it exist today, let alone being on by default. Mouse smoothing is an artifact from the past to try to fix issues with ball mouse. It solves nothing on modern computers, it only causes a bad user experience.