what does "overcompensation" mean?
Compensation for power factor means adding some capacitive reactance to compensate for the usual inductive reactance. 

Fixed capacitors means that you may have to pick certain discrete values so you can decide to leave the load as somewhat inductive (undercompensated) or capacitive (overcompensated). If the load inductance varies during operation  then again you may have to pick some intermediate value and the cancellation may be fairly imperfect.