Kindle Books More Expensive on Kindle Device than Amazon Web Site?

Browsing some books on my Kindle today, then later on my iPhone, I noticed a discrepancy between the prices on the two devices. I first wondered if it was because I was logged in to Amazon UK with a different account (I have two accounts; long story). But I was logged in with the same account.

I was looking at an edition of Plato’s complete works, which retains for £36.56 in hardcover. On my iPhone, the Kindle version of this book shows up at £18.69. On my Kindle, it costs £28.72.

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I looked more closely on the Amazon UK website on my Mac. I found that there are two different Kindle editions, one from 1997, and another from 2011, both from the same publisher. The later edition is cheaper. But the Kindle only shows me the older, more expensive edition. Very odd…