Not necessarily. Sometimes the subsequent sets, as Adultery noted, are on a different board (this may be due to a change in manufacturer for the board, however, other than instructions that may have been needed for a different chipset, no funcitionaly changes in the ROM code itself). Some may be as you said - an upgrade in functionality. Keep in mind that the original ROM set is defined as the original working set. Subsequent sets may not necessarily work in the version of MAME you are using if at all. A good definition of Original ROM Set, Revisions, BIOS ROMS, Clones, and Bootlegs can be found here - see the Game Data section of the article.