"The girl and boy each have male and female sex chromosomes — in other words, each twin has some cells that carry an XX pair (female) and some that carry an XY pair (male)."

They are both genetically ambiguous and present differently due to hormone generation rates.

You’re right, I’m wrong.

Interesting. This would imply that they're both genetic chimeras, as they don't carry a uniform genotype throughout their cells.

The female's sex is varied in the sense it differs from the standard female (gonadal dysgenesis).

Around 1 in 100 humans don't fit the standard male and female definitions according to http://www.isna.org/faq/frequency