It is an observation to which I so far have not seen a proof that this is wrong. In my opinion it also makes sense. The two wing nut is much flatter than the single wing nut (mainly because the single wing nuts have the wing bent upwards and are much longer than the arms of the two wing nuts). And if you have a bar on the scope you have much less space between the nut and the scope to tighten the nut. So to despite of a recoil bar allow tighenting of the nut you'd need to make it flatter, but if only a single wing it might be too tough to properly tighten it. So the solution was to add another wing.