i know this topic has been discussed in the past, but i dont think this answer has been posted yet-it just occurred to me while watching what this could possibly mean-i remember a while ago there were arguements about it, and it just occured to me: after the remaining Primes (not the original 7, but the descendants) saw what had happened with the Fallen betraying them, the decided that all future Autobots would be built to not be perfect, not solid, not...so awesome. The Decepticons only knew the Fallen, and I doubt he would share his designs with lesser beings. The autobos could have made themselves like primes to destroy the decepticons with ease, but then had the same problem they had with the Fallen. but to preserve their lineage, they would preserve it with certain bots. like Optimus. but despite his awesome strength, they would make him not as resilient, in case something went wrong-which is why Optimus could beat the Fallen, and how Megatron, who i clearly not a prime, could kill Optimus. another thought is that when they say "only a Prime" they mean the only good guy strong enough to win is a Prime-however, Megatron, who isnt a good guy, may have been strong enough to beat him, but wanted to learn more from him first. waddya think? obviously not cannon, but plausible?