If they ask why they get paid less and it's because they perform less and they understand that, it might hurt their feelings but at least most well adjusted people can accept it and even better work to improve themselves.

What is much harder to accept, and builds resentment and apathy, is something that feels arbitrary or unjust.

There are co-workers that everyone (including the person themself) know are better or worse than others on the team. The sticky points arise when two people who view reasonably themselves as peers get ranked differently during performance reviews. Then it's right back to arbitrary and (possibly) unjust decisions by the boss.

Perhaps the more productive worker is more productive because their work is not high-visibility. Maybe it's the arbitrary luck of having been assigned a more successful project.

They can’t be paid less for the same job title or job description.

That is not remotely true. There is no law saying everyone with the same job title needs to be paid the same.

Idk what you are talking about but not paying equally opens up a huge amount of discrimination liability.

https://www.dir.ca.gov/dlse/california_equal_pay_act.htm

That is not at all what that law is saying. You can absolutely pay differently based on "bona fide factors" such as experience and job performance. Do you really think everyone with the title "software engineer" at a huge company varying from 0-20 YOE is going to be paid the same? No, they will be positioned differently within a salary band based on experience and job performance. You have clearly never managed a large team or organization and done comp planning for them.

People can reasonably disagree on what the law means. Just because comp planning is done one way or another doesn't mean it is legal. It also doesn't mean that what you think are "bona fide" factors are actually valid.

https://www.costanzo-law.com/google-hit-with-lawsuit-over-cl...

You deeply misunderstand employment law if you think it prohibits people at the same level from being paid different amounts.

It's illegal to pay differently on the basis of sex, race, or other protected classes. It's perfectly legal to pay differently on the basis of performance or anything that isn't a protected class.

The vast, vast majority of workplaces in America pay people at the same level different amounts.