I'm not understanding either WMittens - you brought the Asian men up? Was your 20% for like-for-like or as I had assumed, just in general (which could be entirely explained what Free said)?
I think a lot of the like-for-like problem is that it's only such a very small view of the problem - just look at jobs like primary school teaching, nursing, hell, checkout staff - in each case, men advance faster, and are more likely to be found in management roles, while women spend years doing the rest of the roles (1 man at DS1's school. The headmaster. He's been in teaching less than 5 years, vs. many, many years of experience for some of the female staff).
It seems trivial, but for example, when I applied for a job at Asda (new store, just put in a general CV for a job) I was straight away shuffled to a checkout/customer service role. The men were shuffled towards grocery and shelf stacking. The shelf-stacking roles had greater scope for overtime, a higher starting wage, and because they were also performed outside of opening hours, a bigger chance to get salary multiples for unsocial hours.
My previous job had been at a PC World, where I was the only female member of sales/technical staff. The only other women working in the store were, yup, all on the tills - I significantly out-earned the other women for the same hours because I got commission.
Like for like would only give you a good picture if everyone had the same shot at the jobs.