The PS/2 port is not on a bus that can discover all devices connected to it. i.e. it is not Plug-n-Play. Therefore the OS relies on the system s firmware to find all the devices that would otherwise be undiscoverable. On modern x86 systems, this mechanism is ACPI. However, older OSes such as Windows 95 used a different firmware interface called PNPBIOS to do the same thing. the PNP0F13 pnpid is a carry-over from PNPBIOS. The ACPI prefix is there to indicate that it was enumerated by the ACPI bus driver. The Windows driver model hides such details from the mouse driver -- it can just list the pnpid in its .inf, and get an AddDevice irrespective of what platform enumerator its device was found by.
Of course, PS/2 mice existed before ACPI and even Win95. Back when you used your mouse on DOS, the device driver just assumed the port was at a particular io or memory location, and probed those itself. Fun!