I have no idea 
I never noticed this before. I’ll look into it. It might be a timing problem.
So you want to say that the library measures 100 but it is actually 81.3rpm.
Is it the radian / RPM conversion problem maybe?
If you have some time it would be super helpful if you could try few different velocities and see if it is the same. Also if you can test it with Arudino devices that would be great.
I am going to dig into it as soon as I have a bit more time.