Behavior between different emu/romset?

I was wondering how different MAME versions behave with different ROM sets?

Right now, on my PC at work, I have installed both, a v165 of MAME and a v170 of MAME together with a ROM set of v165.

MAME v165 shows me around 31.750 games available
MAME v170 shows me around 9.100 games available (although that is MAME FX, if that makes any difference)

That's quite a big difference.

So is there a difference using a lower version of an emulator with a higher ROM set, or the opposite, like running MAME v170 with a lower ROM set of v165 (like me at the moment)?

Next week I'm gonna bring my v170 set, and copy it over to the ROMs directory, so that I can compare again.

It's to do with the way the roms are arranged. With older sets there was a lot of duplicate data in each game set. Over time the developers have been reducing the amount of duplication by sharing common rom files used across multiple arcade machines. In theory this means that whenever you use an older rom set with a newer version of MAME it will find less complete roms it knows how to run.

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