Do you notice a similar imbalance between any other of your muscle groups, in particular your lats? My gut reaction would be that you don't do a proper mix of unilateral arm work (like single arm DB extensions, Tate presses, DB neutral grip presses...). However, sometimes this problem can be due to something like a tight lat and tight tricep. A tightness in either of these 2 (especially if in both) can limit their engagement throughout the full range of motion. If so, then implementing proper foam rolling/stretching will help this as well.
This could just be genetic, also.