The fine PWM gain is chosen to be 1/66 instead of 1/256 because of resistor availability. If the feedback resistor on the low-pass filter is chosen to be 22.6kΩ then to obtain a 1/256 ratio requires a precision 22.6k *256 = 5.785MΩ resistor. There is no limitation though if a resistor of this size can be found or implemented.