I can do 20 pullups, why are my biceps still scrawny T_T?

Because being able to do more than 12 pullups trains mainly endurance in your muscles. If you want to gain more muscle and be bigger, i suggest you do weighted pullups, when you put a dumbbell between your legs or put on a backpack filled with sand or something. And when you get to 12 reps with that weight, then put in more weight. It would also help, if you eat a lot. Hope this helps.

probably because you weigh, let me guess, 150lbs?

plus lats are more important in pullups than biceps

20 isn't that many. Get to 100, then brag about yourself.

eat protein. exercise by itself never gives results. eat and drink foods with a lot of protein and other vitamins that promote muscle growth. google it

heh, 20 isn't that impressive :) keeeeep trying! get some weights/go to a gym :o

Pull ups is for back and lats. Do chin ups