Explaining the Gender Pay Gap

This has been one of my little subjects ever since I noticed that people tend to fiddle the sums when calculating the gender pay gap (it's very common, for example, to compare female part time wages per hour to male full time wages per hour).

Today I get a huge boost to my prejudice that it isn't in fact a result of prejudice on the part of employers:

Female consultants working in the NHS are 20 per cent less productive than their male counterparts, researchers claim today.

The pay gap between men and women who all work full time is in fact 17% when measured by wages per hour.

20% less productive, 17% less in wages? Doesn't look like prejudice to me, looks entirely rational.