It is the norm, but this time it's happened to a woman! When David Moyes got less than Alex Ferguson despite being less experienced and less successful, he was a man so it was ok. If Davina Moyes got the same deal as David then it would have been sexism.
The gender wage gap is not a colleague-to-colleague comparison, it is the average wage of full time workers. A female secretary with GCSEs working 35 hours is compared to a male executive with an MBA working 50 hours and the only factors which matter for the comparison are their pay and gender. When other factors are accounted for the difference goes away, so they tend not to be accounted for.
And if those two happen to be married, she will spend more of his money than he does, so who cares where it comes from?