Let me see if I can clear this up. When I was pregnant with the twins, the perinatologist told me he thought the girls were identical twins. This was because they shared one placenta, were both the same gender and had a very thin membrane dividing them instead of a thicker one like you might see with fraternal twins. Remember, we had no early ultrasounds with these girls, which is when it's easier to see if they are close together in a more shared sac or further apart in 2 obviously different sacs. It is VERY rare and extremely dangerous for twins to share one sac completely as their umbilical cords can get tangled in one another and lead to possible death. Our girls didn't share a sac, we know that for sure (I would have been on big time bedrest and would have delivered much earlier) but the separating tissue between the sacs was thin, almost transparent. Which again, led the perinatologist to believe they were identical twins. And we believed him and said they were, because what do WE know compared to him, right??
Fast forward to the letter we got after the twins were born that detailed the pathology of the placenta. Turns out, the placenta MAY have been 2 separate placentas that eventually fused into one placenta. Again, we had no early ultrasounds to show if there were 2 placentas to begin with, so when we saw the ultrasounds it looked like one shared placenta. If there is just one placenta then we could be 100% certain they are identical twins. If it WAS 2 placentas that fused into one bigger placenta that leads away from identical twins and more toward fraternal twins. However, there are exceptions to every rule, as we're finding out with twins.
The pathology report used terms like "fused twin placenta", "a central membrane divides the two placentas", and "sections of the rolled central membrane confirms a diamnionic, dichorionic architechture". Now again, that doesn't mean jack squat because diamniotic/dichorionic (di/di) twins can be EITHER identical OR fraternal, lol! 25% of identical twins are di/di twins:
Normally, twins have two separate (di- being a numerical prefix for two) chorions and amniotic sacs, termed Dichorionic-Diamniotic or "DiDi". It occurs in almost all cases of dizygotic (fraternal) twins (except in very rare cases of fusion between their blastocysts ), in 99.7% of all pregnancies, and in 18–36% (or around 25%) of monozygotic (identical) twins. Dichorionic-Diamniotic twins form when splitting takes place after the third day after fertilization.DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons. 
Most of this has to do with when the egg actually splits, on which day after fertilization (that is, of course, if the egg did split and they are identical). If you're interested, there's more information here.
So DNA testing is really the only way we can know for sure, with a set-up like our little princesses have. Does it really matter? No, it really doesn't. But I want to know what happened inside me! Did I produce 2 eggs? Will I start the process of twins "running in the family"? Or did my fertilized egg split? Plus it's the #1 question we get asked - are they identical? And it really stinks to have to say, "We have no idea!"