Just a theory of mine, with no real facts to back it up, but I think it's more likely the thickness of the individual plates. 6V battery plates tend to be thicker than their 12V counterparts because of space limitations to squeeze 6 cells into the case. If the sulfate crystals can't penetrate to the center of the plate, it's less likely to get a terminal case of "won't re-dissolve". Just a W.A.G. but it makes sense to me.
Jerry
