Some years ago, I treated myself to some nice Sophia Webster shoes.
I'm a tall man, my feet don't fit in any of her shoes, but a colleague had pointed out that er, they're for looking at, and if they're on my feet it's actually harder for me to look at them, so, why not buy them and put them on a display shelf like I would a nice sculpture. That made good sense. I bought an arbitrary size that was available - it's way easier to get the shoes you want if they don't need to fit.
Now of course as a tall man, even though it's not unknown for me to buy heels (in a size I can actually wear) it's rare, and I don't buy skirts, have no use for a bra and so on. But still, advertising isn't very smart, so it's not a huge surprise that after buying those shoes I got considerably more adverts for stereotypically female clothing and accessories even on platforms that explicitly know my gender and previous purchasing habits.
What did surprise me is how many adverts I got for the exact pair of shoes I had just bought. Not similar shoes from other designers. Not other shoes Sophia Webster has designed - the exact identical pair of shoes, often from the exact vendor who just sold them to me.
And my theory as to why is pretty simple. Advertising doesn't understand time's arrow. If you only ask whether these adverts are associated with my buying the shoes that's true. They showed adverts to me, I bought the shoes. The only problem is that causality is reversed, I bought the shoes and then they showed me the adverts.
This was confirmed to me by adverts the grocery delivery company I sometimes use ran at that time. The adverts specified the exact goods I buy from them, perishable goods which I had already booked a delivery of, saying hey, why not buy these from us. Well, I already did, duh? But so long as nobody remembers to analyse whether my actual purchase decision (not the delivery) was before or after the advert these adverts probably look like they have amazingly high conversion rates and you couldn't have targeted a traditional advert so narrowly, it must be worth it...