If you close that HUD and open F5, your speed is higher than 125MPH ( even if it is 125.1 MPH ), which is indeed over speeding. I do agree though that it can be rather annoying, since there's no margin and it is easily a problem if you over pass the 125.0 MPH mark. In my opinion, this could be solved by making the simulator only pointing out over speeding if the speed is higher than 0.5 MPH OR KM/H over the current speed limit.
You're only counted as speeding if you're doing 1.0MPH more than the limit (so in this case 126!) The game does have a habit of messing up when in 3 digit speeds (so 101 or more as the game actually records 100.0 as a speed just before moving to 3 digit reporting)