I don't know, I think where I work we have a pretty good idea for the value each person brings. I don't know how much they're paid, but I do know how good each person is (including whether they tend to complicate things, to use exciting technologies, etc).
But that’s the entire point you’re missing. The pay is not proportional to contribution or technical skill. It’s proportional to market forces and negotiation skill.
I know what level each of our people is, and levels are compensated fairly evenly. The fact that I don't know exact numbers doesn't mean I don't have a proxy.
Maybe it varies per company.