lvl 80 heroics defense cap?

I don't understand....
How is defense cap for dungeons 535, and for raids it's 540?

level 83 raid boss = 5.6% crit chance
level 82 mobs = 5.5% crit chance?

How does that work?

You'd assume that it's 1.87% crit chance per level above you, so lvl 82 would be 3.74%
At 4% crit reduction in dungeons I haven't been crit either so I'm wondering.....is there some
misinformation in the meta? Have I just been lucky?

It is not like that.
Basic crit chance for equal level opponent is 5%. For each level beyond players, mobs get 0.2% crit chance bonus, thus you must have crit chance suppression of 5.6%+ to be critical hit immune vs raid bosses (level 83) and 5.4%+ to be critical immune vs RDFhc bosses (level 82).