MongoDB and Software Version Numbers

Why does 1.8->2.0 break their own stated versioning scheme?

It just says that the major will rarely change - and that is correct. The major changed only once so far, since 2009. One major version bump in two years isn't that bad, and can even be considered rare, for some values of rare.

Nothing else in their versioning scheme says anything, that would suggest that 1.8 would be followed by 1.10. So far, the version number difference between two stable releases has always been 0.2: they're consistent in their versioning.

There are plenty of projects that work the same way.