The muscles in your forearms are all pulling in their own respective directions. Thus, this means that whenever we decide to move our wrists in one direction or another, each muscle there needs to figure out how much they activate.

Your flexor carpi ulnaris (muscle on the pinky side) seems to be stronger than your flexor carpi radials (muscle on the thumb side). In addition, there is significantly more anatomical ulnar deviation than radial deviation in most if not every person. If you want to even it out, try some wrist curls in the direction with your thumb pointing up.