Yes those discrepancies exist. But they are almost always a result of imperfect information at hiring. No one hired 2 people into the same role assuming that one would only be half as productive as the average and one would be 2x more productive. Had they known that going in they would have hired them into different positions or at least different levels within the same position.
And typically those high performers either get promoted or leave to find higher paying jobs.
And typically those high performers either get promoted or leave to find higher paying jobs.