The New York Times asks the simple question, Why Is Her Paycheck Smaller? :

Nearly every occupation has the gap — the seemingly unbridgeable chasm between the size of the paycheck brought home by a woman and the larger one earned by a man doing the same job.

It doesn't matter where the job lies on the income spectrum: all but a handful fall below the bar of equal pay. The percentage gap is about the same for lower-wage workers, like medical assistants, as it is for higher-wage workers, like physical therapists.

They have a nice, interactive graph which shows the details on the wage gap.

The article states the reason is good old fashioned discrimination but then refers to personal choices.