Please post software questions in the proper forum (which, cryptically and puzzling, I know, is named "Software"
Besides (and that's not your fault - it's more or less the entire servo "world" bend on mis-labelling the signalling), R/C servos do not use PWM (externally).
A PWM signal is characterized by the pulse width being able to be anywhere from slightly above 0% to slightly below 100% of the period (if 0.0% or 100.0%, it's plain old DC)
The R/C servo protocol calls for a signal wherein the pulse can be of a duration of between 1ms and 2ms, while the period itself are allowed to vary quite a bit from the defined 20ms.
A signal with a pulse of a defined duration range, within a loosely defined period is Pulse Duration Modulation (PDM)