It's certainly an annoying behaviour - but my personal suspicion is that it arose from the system's origin as the recommender for a bookstore.

I mean if I read a book by P.G.Wodehouse it in fact IS likely that I want to read another similar (but not identical) book by P.G.Wodehouse.

Quite how they managed to hang onto this quirk is a different question that I can't even fathom the answer to (I don't believe they haven't noticed and I find it very hard to believe this is actually the optimum sales technique for vacuum cleaners!)