PHR higher peak to average ratio

Dear all,

In APS023 (Transmit Power Calibration and Management) it says “the modulation scheme in the PHR has a higher peak to average ratio than the modulation scheme for the other parts of the frame”.
Could someone explain why this is the case? Because as far as I know both the PHR as well as payload are using BPM/BPSK, so how are they different from each other?