Not defending the legitimately douchey things amazon does, but it explains it right in that article :

>In short, an Amazon listing features prices not only offered by Amazon, but by other sellers. Some of these sellers may offer the items at a lower price, but the order will not be fulfilled by Amazon and won’t be subject to Prime’s shipping discounts and faster delivery.

If you're a prime member and logged it, it will prioritize purchases fulfilled by amazon and delivered with prime delivery. If you click "Other sellers on amazon" there will sometimes be sellers that are cheaper with shipping than purchasing through prime.