I just thought that since the Game Boy games followed the formula of half the bosses from the same-numbered NES version and half the bosses from the following NES sequel (ie. MM II (GB) has II and III bosses, MM III (GB) has III and IV bosses, etc.) that if (and that's an IF) this Game Gear one was "continuing" from MM IV (GB) (since MM V (GB) has its own unique planet-based bosses) that V and VI bosses would be likely. That must mean there are some originally-MM IV or V (NES) Robot Masters that could've shown up in at least three games instead of the usual two (I like parentheses).
Obviously there's no hard and fast rule; they probably didn't know if they would've made a sequel after the first one, perhaps the inclusion of MM II bosses was supposed to be a surprise and they didn't originally plan to have staggered rosters for future sequels or whatever.
After all, I put up Xtreme 2 maps last week, and noticed that it has some bosses from X, X 2, AND X 3.
Capcom can do whatever they want. No point trying to make any particular sense out of them. (Oh, Resident Evil 4 sold best on GameCube and the Wii version also did much higher than expectations? Awesome, therefore let's NOT put RE 5 on a Nintendo platform! >_<)
ANYWAY, I never did play the Game Boy Mega Man games. When I decided I would make time for them, I was waiting for the Anniversary Collection for the GBA that never did come out as far as I know.