Arcade series?

In Mega Man

Heh, I was reading some of the old topics here earlier... What's the deal with people considering "arcade" to be a separate Megaman series? It's just two games that continue the original series, boss-rush style. If you're going to call that its own series, why not have "handheld series" or "Sega series" or whatnot?

People are ooooooons...

Other than that, I assume it's because there was more than one game with the same distinct playing style.