This article implies that it is Amazon that is changing the prices. Many people don't understand this, but Amazon is a marketplace where sellers compete to sell products. There are currently ~120 sellers with offers to sell a new iPhone 5s (space gray) all with different prices and Amazon just picks one to be the default. So this graph is really showing something more akin to a market price for the iPhone 5c.
http://www.amazon.com/gp/offer-listing/B00F3J4B5S/ref=dp_olp...