That's usually how it goes with arm dominance, I don't know much about why our brain picks a certain hand to use over the other but I do know that's pretty normal. @ your non dominant arm tiring before your dominant one
It's kinda funny because as a child I was always left handed but my mother (who's right handed) thought you could only be right handed so for years she taught me how to write and work with my right hand instead of my left.
Even during weightlifting I've noticed I have far more pushing strength with my left compared to my right even when I do the majority of work with my right such as writing, lifting, gripping.. so on so forth. My left arm tires much more slowly then my right arm aswell. I'm not formally educated about the topic but it almost makes me wonder if our brains just receive signals differently for both arms no matter which we use dominantly?
I'unno. It's a showerthought.