I kinda view them as using a modified version of semantic versioning where, if we take A.B.C, the normal roles of A and B are combined into B, and A is the big annual release with the media hype and advertising and shiniest features at the forefront.

Meaning A and B bumps could be breaking, but C is still just for bugfixes.

Sure it's not a great system from the technical side of things, but it shouldn't be shocking that Apple puts marketing first.