Question on dsPIC33F PWM Period and Duty Cycle Calculation
I found the answer to this question. In this MCU family, the PWM timer clock is slower than the duty cycle clock. If a 1:1 prescaler is selected for PWM clock, for each timer clock tick there are two duty cycle clock ticks. Therefore, the factor of 2 is multiplied in duty cycle calculation. 

Duty cycle clock period = TCY/2;
PWM Timer PTMR Clock period = TCY (1:1 prescaler used)

Source: dsPIC33F/PIC24H Family Reference Manual - DS70187E (pp. 14-34 &amp; 14-35)