Well, Rolemaster, Rolemaster 2nd Edition are pretty straight forward.
Then you get Rolemaster Standard System (RMSS), which would be third edition.
After that, it gets a little fuzzy. Is Rolemaster Fantasy Roleplaying(RMFRP) just a repackage of RMSS, or are there enough changes for it to be considered it's own edition? I think I would call it a repackage.
I'd also lump Rolemaster Express in as Third Edition, as it was basically a trimmed down version of RMSS/RMFRP
Then there is Rolemaster Classic. It's a cleaned up and reorganized version of Rolemaster Second Edition. Personally, I think it is different enough to be considered it's own edition. So, that would be 4th Edition.
That leaves the current edition as 5th. And from a marketing point of view it might be considered advantageous to have the version number being the same as that other well known Fantasy Roleplaying game.
All of this is just my opinion and it would be easy to justify several other numbers for the edition.