Apache version numbering is similar to what the linux kernel used before 2.6.

In a.b.c, odd b indicates the unstable or experimental branch, and even b's are used for major and backwards-compatibility breaking changes. Backwards-compatibility preserving changed can be introduced in c.

So 2.2 and 2.4 are pretty different versions, while 2.3 and 2.4 are closer except 2.3 is unstable. 2.4.0 is theoretically compatible with 2.4.17, except the latter has a lot more functionality and features.

If all it does is add new functionality, and doesn't modify or break any existing functionality, then that seems to be good software engineering to do in a minor upgrade doesn't it?

Seems valid: MAJOR.MINOR.PATCH, with the minimum changes being increment "patch" for backwards compatible bug fixes, "minor" for new/improved functionality that does not break backwards compatibility, and "major" for any API/feature change that is not backwards compatible. They could of course chosen to increment MAJOR instead, but they don't have to in this case.

(I'm assuming they are following "standard" semver rules here)

why wouldn't it be right? Not everyone uses semver, and apache project version #'s precedes semver by a long while.

It's an optional module rather than changing the core logic (as 2.2->2.4 did), so in that regard it makes sense to only up the minor version number.

A better title would probably be: with OPTIONAL http2 support.

not defaulted as http2 though.