
Okay, my own input on "Amazonfail" here.
As a cataloger myself, I can tell you that it is possible for a technical screw-up to cause the observed result. Without knowing more about Amazon's database I could not tell you how, precisely. (Even with knowing some more, it would probably be obscure to me as I doubt they are running Unicorn, so what I am about to describe is a hypothetical situation based on my own experiences.) On our own it would be something like "misconfigure a report that globally does something to subject heading indexes", which is easy to do if you are not scrupulously careful because there are lots of fiddly little options on many reports even in the GUI; those who venture to home-cook API scripts without first consulting Customer Care, beware!
I don't know if this was necessarily subject-category related (it could be some kind of invisible internal tagging Amazon applies, for instance), but if it was, since many books have multiple subject headings it is plausible to me that something some programmer did somewhere affected only certain titles that had the right "lucky" one or two, even while they happened to share subjects three and four or whatever with other titles, so the effect was that only part of the pool was masked.
NB I am not saying there is no problem here, merely that I think remarks like "nuh-uh, it was awfully selective to be just a mistake!" are underinformed about what is possible to occur with massive book databases.