MAME vs MAMEUI ROM sets
MAMEUI is a derivative of MAME.
Sometimes, MAMEUI is released based on a MAME release with a few additional fixes (commits).
That's why the ROM set of MAMEUI can be slightly different from the corresponding MAME version.
Another possibility is that they may count ROMs differently when auditing the ROM sets (for example: not counting BIOS, devices or CHDs as ROMs).