I haven't seen this only in Mayhem...
Back To The Future 2 also has this kind of stuff, there are some sprites that are much more detailed than the others, and you can clearly see that (much like you can clearly see the difference between the goodguy and the badguys in Mayhem)
But I didn't even know about this stuff of 2 different sprites models and all.. Already helped me a lot..
bah, even I have played Mayhem in Monsterland, and everybody knows I think the C64 sucks and the MSX rulez, so you really deserver a whoop in the ass..
(BTW, I am just kidding about the C64 stuff, I do love the machine, just not as much as the msx