Because they use raw input from the mouse (ie WM_INPUT), bypassing Microsoft's latest changesknot4ever wrote:
The problem is with mouse rate checker, with mousemovmentrecord and DirectInput Mouse Rate(dimr.exe) they show 1000hz.
osu uses WM_MOUSEMOVE and/or the other non-raw mouse events, which are now capped at 200hz. This is what mouserate.exe measures
Hopefully peppy can implement raw input, even just as a wrapper around the existing mouse movement code with appropriate scaling.