How can "mental mass" change depending on sex or race?

As I understand it, the e-meter is supposed to measure "mental mass," which is part of a thetan. So everyone, regardless of sex or race, should have the same sort of "mental mass."

And yet the e-meter is calibrated differently depending on whether the person is male or female, and Hubbard said that “The Bantu doesn't register the same on an e-meter as a white.”

How is this explained? Did I miss something?

How can "mental mass" change depending on sex or race?