What if Joe and I and have the same responsibilities. Just to build working code. However, I can produce 100 working/tested lines of code a day and Joe can produce 150 lines of the same.
Joe isn't entitled to a higher salary because he's more talented?
I can drop your example just with this: maybe your 100 working/tested lines produce more money for the company that the ones Joe wrote. Or the company needs the 250 lines of code to produce any money, without all of them your value for the company is 0.
Measure productivity by whatever metric you want, it's impossible for two people to do the exact same (amount / value) work.
The only way to prove salaries are just is for every single person to have a different salary.