Just fyi, I confirmed this observation a long time ago by looking into the IOSU code. Actually the whole seeprom usb seed acts as one big number. Meaning if the last byte runs over you have to increment the byte before it by one and so on (just like any normal integer incrementation in big endian). There is even a "bug" in nintendos code where the first 4 bytes would also increment, if the number ever gets that big (which I doubt). I can give you guys the IOS MCP position of the function, if you want to patch it to stop doing that, I would need to look it up though.