To better phrase this:
Most likely while the rotor is moving you are sampling the position to find out if it's already time to stop.
For the CPU (PIC or ST6) the AD conversion takes time, and also for the SW then to take a decision. All this results in a delayed 'knowing that the rotor is there'. This gives overshoot in position (a delayed stop, not so much due to the mechanics but due to the ADC & SW).
Items to consider next to straight degrees2voltage conversion:
- overshoot (when you stop, the rotor actually moved still a tiny bit)
Henk, PA3GUO http://www.qsl.net/pa3guo