Why is there a gender pay gap among university professors?

On reading a BBC article about the University of Essex's move to eliminate the gender pay gap among its professors, I started to wonder what exactly is the mechanism for such a pay gap to arise in the first place.
The article points out that, on average, full-time female academics are paid 11% less than men in the UK, with some institutions like Queen's University Belfast and King's College London showing significantly higher discrepancies. I'm sure the situation is pretty similar elsewhere, including here in Canada.
Now, much of the overall gender pay gap is a result of a disproportionate numbers of women working part-time jobs, and their disproportion representation among minimum and low wage earners. But in the example of university salaries, surely, we are comparing essentially identical jobs. So, what exactly is happening here? Is there no standardized pay-scales for professors, associate professors, senior lecturers, etc, which are applied to all who qualify for the position? Are there actually separate scales for males and females (which - call me naïve - I would have thought illegal)? Is it something to do with males staying in the profession longer and accumulating raises? Is it just a matter of males dominating, fairly or unfairly, the higher and better-paid echelons, thus skewing the averages?
I found a Stats Canada report on salary scales at Canadian universities, which reveals quite starkly the gender discrepancy at each and every Canadian university when all ranks are combined, but does not break down the gender make-up of the various different ranks. It does also expose the disparity in sheer numbers of male and female staff members (although again no break-down for the different ranks), and also the large discrepancy in possible salaries within ranks. The report notes that: "many factors can influence salaries, including qualifications and number of years teaching. As well, some universities impose a maximum to the salary range for each rank while others have an open-ended scale."
So, does this mean that male professors are typically better-qualified and/or of longer standing than their female equivalents? If that is the case, are the women in any position to argue against that, or to claim that they are being underpaid? Is the problem, then, one of finding out why women are less-qualified, and/or why they typically have fewer years of tenure? It is at this point that I envision the hoary issue of child-bearing and child-rearing raising its ugly head...
Clearly there are a lot of different competing factors involved in the setting of academic salaries. Which raises the question of how, in that case, the University of Essex was able to equalize male/female salaries, and in what respect exactly they are claiming to have "eliminated the gender gap".