Why would it differ in the RNG, I wonder?

Different versions of the chip (different manufacturers, integrated into a larger IC..) just laid the RNG out a bit differently, I'm not sure what the reasons were.

There's a detailed comment at the top of MAME's emulation code for these chips (and a lot of parameters on the emulator's class constructor): https://github.com/mamedev/mame/blob/master/src/devices/soun...