That's a longstanding Amazon bug. As I understand it, a seller can "hijack" the reviews of another product that's no longer available by registering their product as a variant of the existing one.
I found one obvious one the other day, but after about 15 minutes of searching couldn't work out how to report it without a seller account..
My impression is that Amazon doesn't want to know: good reviews, fake or not, drive more transactions in the short term.