I’m not sure exactly what the question is, however, there are 1800 counts/rev inside the motor so you would expect an average change of 90 counts every 50mS. The motor communicates with the brain approximately every 5mS and then there are other background processing loops that mean the values you will see will have a certain amount of jitter. The speed of the motor running with 6V will depend on the load, so if the motor has no load it may well operate near to full speed. So yes, I think what you are observing is probably expected.