As I continued invesitgating why it bends wrong I decided to try something that I thought might be the problem.....because my arm model consists of two meshes I thought maybe the elbow joint my be too close between both upperl/lower meshes so what I did is move vertices higher on the upper mesh. Well, seems that did fix the problem....here's a picture to show what I mean..




EDIT: Another helpful thing is too bend the arm skeleton at the elbow, regardless whether the arm mesh is straigt.