And despite this little oddity, tennis has otherwise a logical and conventional scoring system..?
Set-based 2-margin games are not unheard of: take volleyball for example. As of the naming of the points (15, 30, 40), there ARE some theories around.
Perhaps those two games were kind of "compromise" between British-French "tradition" (no tiebreaks, always 2-margin) and American "novelty" (tiebreak) at the time of its introduction? The fact that, among the slams, only USO allows the tiebreak in the decisive set may point out in that direction.