Apart from level of education, additional learned skills, years of working experience and previous employment history, and actual delivery for the company, there's another factor: human connection.
As a teenager, I had a temp job for a few months, updating a section of an office's archives. In that company, there were about 60 people, one of whom was a very odd man, a happy sort, a helpful man, who always seemed to come to the rescue - out of nowhere - just when something went wrong, and save the moment. If anyone ever fell or dropped a file, he was there to pick them up. If a light went out, he arrived with a new bulb. If a secretary was overloaded with paperwork, he quickly got things up-to-date for her, when the receptionist needed to leave her desk, he took over, and when a professional needed someone to peruse and compare complex documents, there he was, too. He always had a kind word and a sympathetic ear.
I was confused and asked the office manager what his actual job was, as to me he seemed to flit around the office. The manager told me that, in the following week, that man would be on holiday, and I should come and talk to her again afterwards. When I did, she asked me what I'd observed about the mood in the office. I said he generally seemed to make everyone happy, and that it was rather somber without him.
Now that, she said, is exactly the reason we employ him. He has no manager, no specific task, no deadlines and he does as he pleases. He knows something about a lot of jobs in our company, and can help whenever he sees the need. His salary is worth every cent because, since he has been with us, our staff turnover has dropped significantly, and our productivity has increased. Whenever he's away for a few weeks, the morale drops, and when he returns, not only the mood but the efficiency picks up again. People simply get a boost and feel more motivated when he's around. "Remember him," she told me, "wherever you work in your life, and whenever there's a conflict about what anyone else might or shouldn't be earning. Ask yourself whether you're the person who contributes something to help all the others work better, too."