You can only grow your arms so much without growing the rest of your body. My guess is since you're already training your arms three times a week, that they have reached their fill potential without adding weight elsewhere on your body.
You side like the typical beach/frat boy working out arms way too much, but that's okay. Not everyone wants to be Jay Cutler! But, with that said, the 1 inch=10 lbs is pretty true. I'd say it's closer to 1 inch=15 pounds of muscle.
Pretty much the best advice I can give you is to buck up and start doing some more squats, deadlifts and weighted pullups! Also make sure you're eating enough, your muscles can't grow without fuel. Might have to sacrifice a bit of that six pack!
Oh, and one more thing. Come around here talking about 5.8% body fat and no one will believe you until you post some pictures proving it.