A major WordPress version is dictated by the first two sequences. For example, 3.5 is a major release. So is 3.6, 3.7, all the way up to 4.0. Version 4.0 is no different than 3.9 and 4.1. There isn’t a “WordPress 3” or “WordPress 4” – we’re weird like that for historical reasons.
Major releases add new user features and developer APIs. Though typically a “major” version means you can break backwards compatibility (and indeed, it normally means that you have), WordPress strives to never break backwards compatibility. It’s one of our most important philosophies, and makes updates much easier on users and developers alike.
A minor WordPress version is dictated by the third sequence. Version 3.9.1 is a minor release. So is 3.8.2. A minor release is reserved for fixing security vulnerabilities and addressing critical bugs only.
Since new versions of WordPress are released so frequently – we aim for 4-5 months for a major release, and minor releases happen as needed – we only have a need for major and minor releases. We don’t have bug-fix or “patch” releases you normally see with an X.Y.Z-style version number. Rather, we have an X.X.Y version number.
While it’s a bit confusing, our commitments to backwards compatibility and fast release cycles make it very easy for users to be able to update without worrying. (Which is great, considering the days of the version number are numbered…)