So I'm only guessing, but I think it's because the internal display driver was actually working at 60Hz, but the monitor was just advertising the refresh speed

So I'm only guessing, but I think it's because the internal display driver was actually working at 60Hz, but the monitor was just advertising the refresh speed incorrectly to any connected device as 59.95Hz. The error would accumulate and after about 20 seconds the monitor had to "stop" and resynchronize the clock.