It's better for staff morale if the conditions for earning more are made public so that everyone has a fair chance to give it a shot, like deferring the difference to performance bonuses with clear targets or stating clearly if it's based on qualifications.
Doing it in dark corners and later people finding out can result in a very unpleasant work environment, people working together do require help and favours from each other, so it is much better if it's clear that the next person earns better because of 1,2 and 3, instead of it being open to different interpretations and speculation.