Then there is rounding up or exaggeration, so you are looking at a maximum speed of about 95 – 105 mph true speed, which is highly possible on some bikes.
If you have a Mito or an Aprilia RS, you can realistically get a ton, if your bike is de-restricted. My RS will do an indicated 105 – 110, so that could be a true 100mph. My old TZR 125, fully derestricted, done an indicated 95. If your bike is a 4 Stroke 125cc then your speed, flat out, will be about 65 - 85 mph.
You can increase your bikes top speed, through tuning, but also through gearing. Check out the 2-Stroke Engine Tuning section!
However, the main strength of a 125cc motorbike is not the crappy top speed, but its ability to go round corners very very quickly. A half decent 125cc 2 stroke, with a half decent rider, will batter most if not all big sports bikes along B roads and twisty A roads.