Right -- that's because triplets very often originate as two pairs of twins with one (twin) of one pair not surviving to develop in utero. So you end up with a pair of matching twins and a single fraternal brother (the middle guy here) who makes the triplet.
I had to do a little reading on this...apparently it is possible to have two in one amniotic sac and a singleton in another. I didn't realize that only 1 in 1000 triplet births, produce 3 identical triplets. Who woulda thunk??