Amazon makes it clear that people aren't allowed to vote for their own reviews or comments, period. Simultaneously, though, it has a surprising loophole open to the unscrupulous. If anybody sets up a no-purchase sockpuppet account, he or she would be barred, of course, from posting reviews or comments on Amazon, but would nevertheless be granted full voting rights on the reviews and comments of OTHERS!!!
A self-promoter could set up any number of these phony accounts within a few minutes, and they'd be at his disposal whenever he sought to inflate his own vote totals. This seems the simplest explanation preserving all the appearances and explaining the outrageous vote totals marking the Grady Harp Express these days.
Editor note: you don't even need to amass such accounts: you can create a new one for every 'voting session'. It's that simple: not only don't you need to buy anything from Amazon, you literally need nothing other than a throwaway email address, and even that most likely isn't verified. Takes ten seconds to type in a nickname, email address, and password, and then you're in business.