In my experience (only in UK - but probably the same elsewhere) it all comes down to qualification level.
For example my girlfriend works at the same company at me, and works in process optimisation. This job requires high skill level and engineering qualifications. She however doesn't have the qualifications, but can do the job perfectly well...in fact better than others IMO. (not bias!!). But she gets paid £10,000 a year less.
Partly due to lacking the qualifications, partly due to being female in a male orientated job.