it's a meaningless metric used for marketing, nothing more.
e.g the whole claims that neogeo was '24-bit' because it has a 16-bit 68k and 8-bit z80 (by the same logic the Megadrive is also 24-bit tho, which is where the irony comes from because they were quick to claim that was 16-bit)
most of the video chips found in the systems have 8 or 16-bit buses, even when used with newer CPUs; for example, the Megadrive graphic chip is actually an evolution of the 8-bit one used in the Master System and still operates on an 8-bit bus.
things like Lethal Enforcers by Konami pair up graphic chips typically used on 68k driven platforms with an older 8-bit CPU (really looks like that game was made to clear out excessive parts)
but as I said, it's an utterly meaningless metric, especially with arcade hardware because you see chips from different generations intermixed all the time. What matters is how things were used and what they were paired with.
take PGM, te main CPU is a 16-bit 68000 processor, some of the PROTECTION cpus in the cartridges (used for nothing but protection checks) are 32-bit ARM processors, more powerful than the actual system base system. there are quiet a few examples of this actually, presumably because the game programmers were more familiar with certain CPUs, so stuck with those, while the people working on the protection systems used newer CPUs but barely made any use of them.
how many 'bits' a system is has nothing at all to do with visual quality, and doesn't even tell you much about a system in general.