I use a homeschool curriculum to teach my kids, and sometimes I'll look up books on Amazon to see reviews and more information. It always amuses me that *almost* every time, most of the "suggested" books are also used in this same homeschool curriculum! So I'm sure there's a huge bias for some items because they're on some list or something, and then a ton of people look up everything on the list at the same time, so you end up with a bunch of items that aren't at all similar being grouped under "suggested" because of some list they ended up on (which might even be as disconnected as somebody's favorites, or the like). The more popular/widely-read the list or the more obscure the item, the more likely it would be that the connected items would end up on the "suggested" list.
I did end up with some really creepy books ending up on my Amazon suggested list at some point, and no idea how. And my in-laws had some weird issue with Netflix at one point, where it suddenly decided that if they like historical British drama, they MUST also like S&M, anime, and urban movies, too. And maybe gay cowboys. Because, hey, all *very* similar to Foyle's War, you know? I think it stopped doing that after a while but they never knew what set it off.