Who would buy an e-Book from the iBook store? You're limited to just your iDevices in terms of reading your content. And don't tell me it's because of cloud syncing. The Kindle application sycns all of my content with every device that I own.
It's possible that most of iBook sales are for special applications - for example, for iPads in schools. In this case there is no need to view the content on other devices. The volume might be low but the prices high (hence 20%)