The B7100 doesn't have position control. So, when you move the lever back, it raises until you go back to the neutral position. When you pull it towards the front of the tractor, it drops until you return it back. So, to raise to a determined heighth which is less than all the way up, you have to jockey the lever until it is where you want the implement, then return to neutral. Is that what you are refering to? If so, that is normal for the B7100. It is more jerky the higher you have the RPM's, and less jerky at lower RPM's. It is a design that you learn to live with.
While I can accept it on my tractor, because it is what it is, if I were buying a new Kubota, I would expect position control. It is disappointing that Kubota doesn't offer it on all models- it should be a standard expectation.