Having the same job title and having the same job performance are two very different things. If one person is grossly outperforming another do you have to write them a new job title to compensate them for it?
Otherwise it just reeks of favoritism.
If someone outperforms another person, you have to be able to prove that with numbers. Being upfront about pay does mean that you are upfront about pay differences. For example: Alex has a higher degree or more experience, Robin tends to be finished before the last minute.
Things like job performance are harder to measure unless folks are put in very similar situations. For example, if you measure a cashier's performance by the number of customers they serve, the overnight cashier is almost always going to perform badly simply because there are fewer customers at night.