But a +1 to defense against a single opponent makes more sense.
I actually agree. But the rules are written clearly, and they work. Assuming they mean something other than what they say would be pretty weird.
More on topic, I think part of the reason social stunts might be higher power is because they expected physical conflict to be much more prominent, and therefore those skills would end up on the top of the heap, while social skills would be lower on the totem pole.
I doubt this very much.
First, because writing a deliberately-unbalanced skill system and then patching it with stunts is just silly. Why not write a balanced system? It's not as if they're incapable; in fact, I think they did it.
Second, because nobody sane puts secret rules into an RPG and then doesn't tell the players about them. That's just nuts.
What we're looking at here is obviously an error of some kind. If I had to guess what kind, I'd say that the stunts and the stunt rules were written by different people and changed during the writing process, with imperfect communication.
PS: The problems with Killer Blow have pretty much been covered.
PPS: If your objection is to weapon specialization stunts, then your argument is actually sane.