Basically, it comes down to wind drag.
If you have low drag, you can reach the hp peak rpm of about 4750 rpm in top gear, which is somewhere around 80mph.
If your air drag is higher, then the bike poops out at a lower speed.
Try getting lower down on the tank, in a crouch, and you'll probably gain about 5mph in speed.
Tight clothes help, and flappy clothes hurt.
It's only got about 19.8hp at the rear wheel. It needs all the help it can get.