It didn’t use to be that way; Debian 3.1 was a major release after Debian 3.0, and Debian 6.0 had minor releases up to 6.0.10. This did not change until Debian 7 (which was called 7, not 7.0, and had a minor release numbered 7.1). Therefore, many people might still be used to the first minor version number being a major release, even though it is not true anymore.