The different names for the different "type" of astromech - R2, R3, R4, etc., was a system invented by the Lucasfilm marketing department, not by Lucas himself. He names the astromechs whatever he wants since, of course, like all things EU, he's never paid attention to it. That is why he named Obi-Wan's droids in the prequels "R4," even though they don't have the pointy "R4" dome type, and why, in the new CW series, we've seen astros named R3, R6, and R7, even though they all have "R2" style domes.
I think to him they are all "artoo" units, and the different domes make as much difference in their functionality as their paint schemes.
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