To be a bit more precise on this, P-ID vs. USB device ID:
The USB device ID normally contain the camera's P-ID,
but the reported number may vary dependend on the camera's connection mode.
It looks like the P-ID is reported in the USB device ID always when the camera is
connected in PTP mode; on some older Powershots with mass storage
support or with other additional USB protocols the USB ID number may be different
when the camera is connected in one of this modes.