I'm not the person you were asking but it's happened to me too, with physical books, not e-books. I don't know if I can explain how it happens. I'd say that me not knowing the exact content of my bookshelf is similar to me not knowing the exact content of my fridge and pantry. There have been a few times when grocery shopping where I can't remember if I have a less-frequently-used ingredient (like cream of mushroom soup) but I'd like to make something using that ingredient soon, so I buy the ingredient, and then when I get home I find that I did have a can in the back of the pantry that I bought a year ago and the expiration date hasn't arrived yet. Oh well.

If you can really remember every single one of your nearly a thousand ebooks that you've bought, that's both impressive and baffling to me.

Faced this problem multiple times before. Tbf, I did buy and subsequently transport enough books in luggages to restore a country's economy