Its not an 'indicator' its a rather weak way of determining someone's likely negotiating position.
If I do (as I have) a role that tripled income and generated £60million of funding, but wasn't paid much for it, that's because I undersold my self, and wont ever ever do that again. I can still do that, but did a hobby project, so had a £42 equivalent salary, but if someone wanted me to generate £60million again, and tried to pay me £46k I would rather punch them in the face repeatedly and get a small fine than allow them to ride me that hard.
It also doesn't explain students going from zero to £36k, or someone coming from China to work here, or someone moving from sheffield to London to work in the city either, so its a total bollocks indicator.
I think its nuts to put this as a question - my desired income is around $645 trillion, but I'd be happy for that to be all company profits, with my salary being $8,000 just as much as having a $645 trillion salary as such.
If it were my interview process, anyone who didn't put at least £200k I would think was unimaginative.. 