The Kindle format is proprietary. Amazon has an official tool for converting epub to the Kindle format, however it is illegal for us to use it[1] to generate the kindle version of 99 Bottles of OOP.
Therefore, we've had to generate a .mobi file using an unofficial tool, and the result is not as good as the official one.
It's perfectly legal for you to generate the .mobi yourself using Amazon's tool, kindlegen.