I'm guessing it is a combination of the following factors: carbine buffer vs. standard buffer, 11.5 vs. 16 gas pulse, old style telestock (?) vs. A2 stock, weight, weight distribution, and 11.5 vs. 16 muzzle velocity. (OK, the last one is unlikely to be noticeable.)
If you could legally do it, I would try swapping uppers to see what happens.