Is it possible that actors Cole and Dylan Sprouse are "half-identical" twins?

I'd never even heard of such a thing until I had to do some research on twinning. According to the Sprouse twins' official bio, they're identical. My kids watch them on "Zack & Cody," and we get into this discussion wherein they insist to me, "They look just alike, one's just a little heavier and their haircuts are slightly different." And I insist that they look like fraternal twins, and I'd never get them mixed up, even if they were the same weight and had identical haircuts. In fact, the Olsen twins (who I know are fraternal) look more alike to me that the Sprouse boys. It just doesn't seem to me that environmental factors could come into play much in twins their age who've always been together.