There's probably a nontrivial number of people who renovate houses one bathroom at a time.

That's the sort of thing you buy a new one because one broke and now it shows up how tatty (worn, old, messy) the other one(s) in your house are. Or someone asks where you got it ... "oh, I can't remember but Google ads showed me the same one of available at $diy_store", or whatever.

Its a math mistake on Amazons part.

They’ve conflated “most purchases of a toilet seat are made by someone who buys another” and “most people who buy one toilet seat buy a second”.

There’s a small, but high volume group of toilet seat purchases — eg, office buildings or apartment maintenance.