TPS61169: Why is PMOS at the bottom and NMOS at the top in the PWM control circuit for Vfb?

Part Number: TPS61169

In the PWM control circuit for Vfb in the TPS61169 (image on page 10), why is the PMOS at the bottom and the NMOS at the top in the push-pull configuration, which is different from the typical push-pull driver circuits we usually see?