The timer that's used will only accept int milliseconds, so for example, with RefreshRate=60, we must wait 16.66~ milliseconds per frame, and the timer cannot do that, so it'll refresh every 16 milliseconds which results in 62.5 frames per second.
Other common values and their current outputs:

• 30: Must wait 33.33~ ms, waits 33 ms, outputs 30.3030~ fps
• 75: Must wait 13.33~ ms, waits 13 ms, outputs 76.923076923076~ fps
• 144: Must wait 6.944~ ms, waits 6 ms, outputs 166.66~ fps