No, it's a single SET which consists of various roms. yes, MAME is updated montly and yes, romsets do change from month to month, not all, but some. Reasons can be multiple: bad dumps get replaced, new dumps (of other ICs on the board) get available which may not directly influence game play but are needed for consistency), romnames change to match actual IC on the board, sets are reordered, device roms are separated etc etc...
so all you need to do is audit your set in question.
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