Inaccurate motorcycle speedometers

While I’m preparing some new content for next week I thought I’d express my frustration at one aspect of motorcycles. 

Why are motorcycle speedometers so inaccurate?  I have never ridden a motorcycle that shows the even vaguely the correct speed. It’s noticeable and annoying since traffic wizzes past when I’m going the indicated limit. Surely not everyone is speeding! It gives cars drivers one more reason to get annoyed.  Sure speedometers are not 100% but this is silly. 

Based on the GPS the Hayabusa is about 6kph increasing the faster you go and the DN is less than 5kph most of the time.