A P-n-P monitor selection will only allow a max refresh rate of 85hz, even if the monitor is capable of higher.
Mfg. monitor drivers allow the monitor to operate at it's maximum capabilities. If it CAN do 100hz, that option is exposed only via the monitor software. Sony monitors are a great example of this. My G520P defaults to a max of 85hz in windows when I set it for this, but after I load the Sony driver, viola! Now I can get to those refresh rates over 85hz like magic.