Ever see the smaller skinny guys who beat these monstrous behemoths? I guess it's all about leverage.
My friends brother's an NYFD fireman who does it as a "hobby"...the dude is a beast and travels the country in comps.
His last one was in Vegas and was beaten by some normal average beta...had to be a humbling experience to say the least.
It's a different kind of strength - tendons and explosive/holding power. Leverage and form also are important. Think Olympic weightlifting. Big muscles have very little to do with being a successful arm wrestler. Of course, all the top guys do incorporate weight training, but not in a bodybuilding sense.
Bench press specialist Scott Mendelson recently started arm wrestling and even though he is very strong, he gets smashed by many guys smaller/weaker than him.