its completely normal, geneticaly our arms are differently structued, if you take a good look at both arms and hands youll notice their both slightly different in many ways
its just genetics, my left bicep has more definition in it rather as my right has more mass
same goes for my shoulders, its just genetics, u are perfectly symetricaly structured
others thing sot take in factor is which arm you use more, since this will cause differnce muscle fiber compostion
theirs also daily factors to take into place, aswell as lifting techniques snd trsaining, e.g the way u hold the barbell when you deadlift will effect the structure of ur bicep since ur holding it with a different grip
theirs to many factors that come into play here, in the end of the day thiers very little you can do to change a slight difference in ur arms