@Webwolf Version numbering for software, when you're not actually working on the software itself, generally feels something akin to witchcraft and magic, which is to say you shouldn't try to apply normal logic and thinking to them. It generally doesn't work. Numbers get skipped and usually those of us outside who are not major nerds have no clue why.

Sometimes its due to an internal version that while it served an important stepping stone, was never intended for release. In this case, I imagine that 3.3.4 was where they merged in the code for Battle.net 2 from the cataclysm base, while 3.3.5 is the release version for both BN2 and the Ruby Sanctum.

In other cases version numbers can be more.. symbolic. For this example we look at the labeling for the Gnomeregan/Echo isles event patch, which is labeled as 3.9.
Obviously there is not 5 internal major content patches between 3.3 and 3.9, but the thing is, 3.9 in this case doesn't mean "After 3.8" it means "Before 4.0".
With other expansions blizzard was able to roll out world events, game systems updates (Talents, honor system etc) and the actual expansion content all in one package, an x.0 patch, as the expansion content was always behind closed doors. With Cataclysm however, they can't do this, you can't have the world changing update before the world event leading into it, so we get 3.9, or "B44".