I can believe it. I have a friend who is 5'6 and when he is 169 he has 18 1/2 inch arms, along with massive everything else, of course his bodyfat is well below 8%. He can also bench 405 with perfect form at that weight. He is now 200 with very close to 20inch arms, non pumped. Obviously he has great genetics, but maybe this is also the case here?
I've seen some strange things at the gym. One of the best, is this 6' kid who is very shy, 171lbs. He came up to me for a spot one day. He loaded 315 on the bar, I thought, no way, I'll be doing heavy upright rows! Well, he sure didn't need my help, he benched that weight, and then some.
I guess my point is, sometimes theres that genetic freak out there, or someone who is just naturally strong, has naturally big arms, etc. When I was squating only 225, I had 28inch thighs. It's just a gifted body part of mine. Let's not be so quick to call bs.
As for the original poster, I think your training is terrible. Stick to mass building basics, and eat, eat, and then eat some more. Whole milk, whole eggs, peanut butter by the spoonful along with deadlifts, dips, chins, squats, etc, etc will only do your body good. Good luck.