They are different quite a lot. The R5 is produced in a different way. The R5s bottom bracket is included in one piece with down tube/head tube/top tube piece.
R3 is a different story, the bottom bracket is bonded to the down/head/top tube piece later....so, additional material has to be used to "connect" these 2 parts.
Of course other tubes are layed up in a different way with better materials.
Just curious ....what exactly is the source of this info? I have not seen this sort of info revealed anywhere. Are you confusing R5CA with R5?
On another note, I think this thread needs to be shut down...too much misinformation on weights with people posting weights with frame accessories that should never b included in frame weights. Simply too much misinformation resulting in apples to oranges comparisons of weights among different model years.
If u r going to post weight of frame do so without anything hanging on it...no bearings, rear mech hanger, seat collar, cable guides. Without a consistent baseline with which to make comparisons weight info is useless!