I think false positives are left because it's impossible for a seller to leave a negative, so they click on the "Positive" button, then give a true reflection of the transaction.
I'm having a bad run at the moment, including a buyer who obviously decided she no longer wanted to buy the item but wouldn't reply to my emails - not even to confirm what I suspected - that she'd changed her mind about buying it.
I've just looked at her profile and see the last four positives say something like "bought a month ago but did not pay and won't respond to emails". All four are from the same seller. I can understand why the seller has done it, even if it's not the way eBay want things to be done.