Its true that d20 books generally do better than John's example at the launch of the title but there is a secondary problem happening in d20. The problem is the restock and shelf life absolutely die after the first two to three months. It reminds me of comics. You better sell the majority in the first 90 days to make your money or expect a long slow crawl to turn a profit, if ever.
What is turning this around for us a little bit is the longer shelf life cycles in the book trade. But only a little.
Ars Magica still sells better than Harn, which is even more snobbish and unpopular (and poorly distributed). Who wants to play a peasant farmer? Those self-proclaimed "Harniacs" should get a life!
Oh, wait--I'm one. Nevermind. Great article, though!