Two possibles spring to mind.
If someone fitted a 16T gearbox sprocket from a 700 on your 750, which should have a 17T, then the gearbox will be running 17/16 times as fast as it should. The speedo is taken off the gearbox shaft so will show a higher value than the bike is actually doing.
The other one is that if yours has had the instrument pack recall change done and a DCT item is fitted instead of a manual (if this will actually work, I have no idea, just thinking aloud), then it would be a similar issue with the speedo calibrated for a 17/39 ratio and getting a 17/43, i.e. gettin