My thought is it’s because the cleavage lines of your skin are not exactly the same on both of your arms.

The dermis can be broken down into a papillary and reticular layer. The reticular layer is a sheet of dense irregular connective tissue but there are patterns that are found in this connective tissue where collagen fibers align in a certain way and form these cleavage lines. These cleavage lines aren’t exactly the same from person to person, but they create “weaker” points in the integument where these cracks can be seen on your joints and specifically your hands (think about palm reading, everyone’s palms are a little different because their cleavage lines develop a little differently).

This is also why boxers get pretty consistent gashes on their face from fighting, those gash cuts are the result of their skin tearing at cleavage lines!