The power output can get lower with the state of charge because of two reasons:

Limit on the output current. Cables, heat dissipators, fuses and battery performances are related to the current. So it will be smart to restrict the current draw so you can save weight(read money also but mostly weight) on the components. So with less voltage and same current, less power.

Motor Kv is the ratio of RPMs vs Voltage and is constant under load. As the voltage of the packs gets lower the motor can turn at less speed.
Example: With full 120V battery the motor turns at 4500rpms and on near-empty state (85V), it will turn at 3200rpms.
You can arrange everything to make the motor turn at max RPM on any state of charge, but you will have some high-frequency energy losses and black magic stuff like that.