G'morning,

what you didn't note was the gear you have on your characters. i don't know about anyone else, but as my groups advance, my main usually gets the good stuff first... i do this hoping he'll live longer (i should make him last... as he'd probably aggro less... *shrug*)

anyhow, check your gear and specs and see if it's EXACTLY the same across all characters.

and no, key passing has zero to do with numbers produced by blizzard on server side...
unless they give a higher chance to crit to the first attacker...