They are different because people invent words. If the definition of gender isn't useful to you, which I claim it isn't, then stop using it. Just say the word sex and we can stop having debates over a silly word that has always had an engineered time bomb definition.

I'm not interested in what anyone's gender is because it's a useless form of categorization that doesn't correlate well with a description of the real world. I'm only interested in what someone's sex is.