I never challenged this, because the basic math is (again) level + 12 + stat. The "+ stat" provides a considerable amount of variation, if the author even realizes that this is the case. The negative modifiers you see is because some monsters have terrible stats, but generally they are pretty consistent.
You are mistaken. Monster NADs are a set formula, exactly as I've described. The addition of stat mods is what causes the strange variations between creatures. But it is not correct there is not a set formulae because there is. See pages 184-185 in the original DMG. If you disbelieve, open the monster builder and just increase a creatures level. You will immediately observe that the NADs all rise uniformly by level + 12. To get the variations seen in most monsters, start modifying the stats of the creatures. I have to concede, this is possibly the most poorly understood aspect of making monsters in 4E. The concept of adding the monsters stat mod is just buried within the text, but not clearly stated anywhere. Not their stat mod + 1/2 level though, some of the monsters with silly nads are because the author didn't realize they shouldn't add stat + 1/2 level. Whoops!
It varies. Use the monster rules in the DMG with Level +12 for all roles.
Take any monster. In this case, I'll pick two controllers:
Meazel Strangler Level 13
Average highest ability score: 13 + 1/2 level = 19
Fort 24 (with Str 16, stat -1)
Reflex 26 (with Dex 20, stat +0)
Will 24 (with Cha 16, stat -1)
So, Level + 12 = 25. Reflex is high by 1. The DMG rules state to go 2 higher or 2 lower than the average. Since Dex is 4 higher than the other 2, it does seem reasonable for Reflex to be 2 higher than the other 2. This one is close to being reasonable, but doesn't precisely match the DMG rules.
Blue Arcanian Level 10
Average highest ability score: 13 + 1/2 level = 18
Fort 22 (with Con 17, stat -0)
Reflex 21 (with Int 19, stat +0)
Will 19 (with Wis 13, stat -2)
So, Level + 12 = 22. Reflex is low by 1. Will is low by 1.
Int is higher than Con, but Fort is higher than Reflex.
If I go into a different role:
Derro Warrior Level 13
Average highest ability score: 13 + 1/2 level = 19
Fort 24 (with Con 17, stat -1)
Reflex 27 (with Dex 22, stat +1)
Will 24 (with Cha 14, stat -2)
So, Level + 12 = 25. Reflex is high by 1. Will is high by 1.
Con is 3 higher than Cha, but Fort is the same as Will.
If there is a modification of the general equation that changes the numbers to what they show up for monsters, please let us know.
Is it based on role for different defenses? Not according to the two controllers above.