The two wheels can't be turning at different speeds unless one is slipping.
Maybe try reading this again:
The speed values may be different because of small errors in one or both (due to tire rolling radius not being exactly as expected, for instance). The speeds will also be different whenever the bike is turning a corner - the front goes further than the rear unless the rear is slipping significantly.
The will be turning at different rotational
speeds if they are not identically sized (and they're not in most motorcycles, and not in this case). They will be running at slightly different road
speeds (which can be ignored for speedometer purposes) if turning a corner or if either driven wheel (two in this case, which is not normal for a motorcycle) is slipping.
On motorcycles, OEMs use the front (not the rear as suggested) wheel for the speedometer.
But in cars, OEM's use whatever wheels are driven for a mechanically driven speedometer, regardless of whether they are front or rear, although I'm sure there were some weird antique things with a speedo gear on a non-driven front hub and a long cable to the speedometer. The difference in speeds is negligible for the purpose of a speedometer. With all new cars now having ABS they have a speed sensor for each wheel, and it's anyone's guess what speed is used for display on the instrument panel.