It has to do with assuming that the current in the power supply path is approximately related to the average motor current, even though the power supply is being connected and disconnected from the motor at PWM frequency. So there is a lot of high frequency energy passing through the supply leads, unless there is sufficient bypass capacitor energy storage right at the H bridge to supply the pulses, and the supply leads only carry the average current to this local storage. If the bridge is not sufficiently bypassed, then you have to account for the inductive effects of the supply wiring, as well as the DC effects, taking the current pulse wave shape (frequency content) of the supply waveform.