That's the encoder we gave you
.
The getrate from the library is an instananeous rate. You'll be better off using the 'get count' function, and then measureing the time between loop updates like this:
(New count - old count ) / loop time
If that ends up still being noisy, you can average, or calculate over more than one loop. Also, make sure you're running in single sample mode rather than 4x to remove phase errors.