Why the right button reads slower
Nearly everyone scores lower with the right button, and the reason is anatomy rather than hardware. The index finger rests on the left switch and does the work of pointing; the middle finger sits on the right switch and has less independent control, so its presses are less even and its recovery is slower.
A gap of 15 to 30 per cent between your left and right rate is normal. A much larger gap usually means the right switch is stiffer, which is common on mice where the left button has taken years of wear.