I discoverd the same behavior. I also found using a Dimmer item not so handy. Most use cases are 0 or 100. If you want a specific high it is easier to start a full move and stop at the needed position.
At least that was the case for me.
So i used the rollershutter item as a proxy item. A rule catches the commands (UP/DOWN/STOP) and send them to the correct channels (LEVEL and STOP). In this way i also could translate UP to 99 instead of 100 to go around this problem.
You can find my rules etc. in the allready linkt post.