If youre lifting a barbell, one arm can't lift 60% of the weight and the other 40% if they're starting and ending point is the same. They both go through the same motion, and travel equal distances from the starting point to the ending point, lifting the same amount each time.
One arm may be stronger, and the other will fail before it, making it unable to finish another rep. But since you can't finish another rep with just the strong arm, they are still doing equal work.