It's not like there's a 'black' gene and a 'white' gene, and you get one or the other. Lots of genes control skin tone, some making it darker, some making it lighter, and we generally have a mix of each. The sperm and the egg could have randomly had more of one kind and fewer of the other, so when they fused, one girl ended up with a lot more 'lighter' genes and another with more 'darker' genes. They'd still have some of each, just in different proportions.
It's like, in interracial couples you might get one kid who's a bit ligher, one a bit darker. This is just more impressive because the difference between them is greater than normal.
Quite frankly I'm more shocked at the name "Remee".