I don't know how Jagex does their updates, but I imagine that they run each set of new content through on a separate server that JUST has the areas needed for the quest/skill/update or whatever, however much is needed. Then they later add this on to the full game in the week leading up to release, which then needs to be tested for a whole lot of other stuff. The game changes week by week, and usually a couple of times during a week too for a small bugfix or whatever. They have to compatibility test AFTER the previous update has hit the live servers otherwise they would have to know what order they're releasing the updates and every single small change months and months in advance. I imagine that's the reason why updates are delayed, not because they don't test them but because of the nature of the game and the way it's updated.