[USFRIENDINVICTORIA]

A tiny fraction of people have different chromosomes — XXX, XYY, XXY.

[fr_freak]

A tiny fraction of people have different chromosomes — XXX, XYY, XXY.

Yup, and those are properly called "birth defects".

[Svartalfiar]

A tiny fraction of people have different chromosomes — XXX, XYY, XXY.

Yes, and people with Klinefelter's or Turner's or other such genetic abnormalities are still definitively male or female - If a 'Y' is present (XXY, XYY), it's a male. No 'Y' (X, XXX, XXXX), it's a female.

There are really only two real cases that should cause confusion in these cases: true hermaphrodites, and chimeras.

Hermaphrodites physically develop incorrectly, having both male and female reproductive parts, and the one that ends up being the main one visually may not match what their chromosomal makeup is.
A Chimera is someone who's absorbed another human into themselves, such as a twin in utero. While they are definitely male or female, with parts to match, many of their cells will have the other twin's DNA/chromosomes. So their DNA test may sometimes not match what they actually are.