Personally, I find it a bit troubling they are not following their own guidelines (and rules) and conjuring numbers out of thin air. I had enough of that when I played 2E (how many XP does a rust monster award? there were 3 different values according to the source/formula you checked), and was happy when in 3E everything seemed to be done correctly. Now with 4E it seems they have gone back to the practices from the 80s. If the rules of a game system do not accommodate what the designers want to do with it, then the system isn't robust enough.
Things like this is why we never can have anything nice!
Simply said - they could just stick to the simple DMG guidelines and every monster will work the same. It has a level, a role, its expected attack and damage, and that's it.
But the thing is we as the players and WotC as the designers want more.
Stuff like Zones, Area Effects, Petrification, Immobilization, Darkenss Auras, deadly stench, domination. Mind Flayer Pet Slaves, Medusa Hunters, Cold-Blooded Assassinss that strike from stealth.
So, we start with the baseline, and then decide: "Hmm, being able to immobilize every other round and shift away?" "Sounds like the damage or the AC should be lower then."
"Hmm, a race enslaved by Mind Flayers - you bet it has a really low Willpower!" "But add some psychic resistance, gained by surviving mental attacks from Mind Flayers."
The guidelines are the starting point, not the end point. The system would not be robust if it couldn't handle this. The system would be no fun if it wouldn't allow this.
