First of all: I don’t know.

But here’s my two hypothesis:

• girls can detect other girls by smell (pheromones or something like that?) and by taking HRT you start producing that same smell (unlikely, but funnier)
• girls (both cis and trans) use hygiene products that smell nicer (and “girlier”). Most shower products “for girls” smell like fruit or tea or something like that, while products “for boys” smell like car deodorant or chemicals.

Hormones affect the scent of your body odor. It’s hard to put into words what the difference is other than testosterone makes you smell like a guy and estrogen makes you smell like a girl. I suppose testosterone scent is… more pungent? This is with proper hygiene btw.

It was actually my biggest source of dysphoria when I fell off my hrt for a bit. When you stop smelling a certain way for over a year you lose any noseblindness you had. I could smell myself constantly and I hated it. Hair products and stuff can shift the scent but it doesn’t really hide it completely, unless it’s some really strong stuff, in my experience.

Side note, I’m reasonably sure this is why my cat at the time suddenly became more affectionate, he didn’t really like men so when I stopped smelling like one he stopped perceiving me that way. This was well before any visible changes occurred.