In my life, I've only ever personally known two sets of "twins" - my dad and his late bro, and my friends HB and MB. I've long suspected that the labels of either "identical" or "fraternal" did not apply entirely to either set of twins, and perhaps now my suspicions might be confirmed.
According to this article, a new twin variation has been identified, where there is only one egg but two sperm who have managed to penetrate into the egg prior to dividing up into two separate embryos. In the past, they believed there were only two ways to create twins: two separate eggs and sperm (fraternal), and one egg and one sperm that divided into two whole embryos (identical).
Personally, I believe in this semi-identical twin theory. In the case of my dad and late uncle, they looked far too much the same in their early years to be fraternal, and yet they were considered as such. In the case of HB and MB, they looked way too different to be considered identical, and yet the evidence suggests that they are genetically-identical twins.
So now, this proposes a third theory into the mix. However, it may not be a perfect theory in that it has yet to answer how the DNA then matches up; if you're not entirely identical, does that not make you automatically *different* and therefore, essentially fraternal anyway?
I'm no geneticist so this is way beyond my realm of understanding, but it is nice to think that perhaps there is more to my long-existing suspicions than just "gut-feeling." And even if it doesn't lend total support to my personal theory, just let me live in my little bubble anyway, okay?