While many OSS have adopted semver (http://semver.org/) it is not hard to find one that it doesn't. I'm sure the maintainers must have its reasons (I hope it's not laziness), and would like to know them.
I don't exactly know but on what project do you base yourself on?
I can see many projects not versioning at all (or just tagging incrementally when they find it's stable enough). So my guess is that semver wouldn't be followed because they do not keep track of the features and breaking changes that might happen.
