I remember reading on one of these threads that all LiPos (no matter what brand) are actually produced from only 3 plants in China, and there are apparently regulations for the chemical makeup of higher capacity LiPos.
I think (and I could be 100% wrong) that one of those regulations requires batteries over a certain capacity (say 5000mah) to be 2s2p. So I think any battery over that limit, no matter who sells it, be it Turnigy or Gens Ace or Reedy or Team Epic, etc... has to be 2s2p.
I don't remember where I read that, but I don't think I could just make that up outta nowhere.