Here's what Amazon's Customer Service told author Mark Probst who writes YA gay fiction not erotic and not adult.
In consideration of our entire customer base, we exclude “adult” material from appearing in some searches and best seller lists. Since these lists are generated using sales ranks, adult materials must also be excluded from that feature.
Check out what SmartBitches has to say about this so-called Amazon Rank and check out #amazonfail on Twitter.