each random ass has 10 different difficulty parameters ranging from 0 to 10. Each time a new mission is created by the random ass npc, he rolls a number between 0 and 45. 0 is F and around 45 is A. Why 45? because I set him to roll 45max. Now, each time he rolls a mission, the difficulty points are distributed randomly across the 10 params.
Now, you might roll a nice E rank mission with all its 10points allocated to "Mob threat", resulting in an npc ppol with 300hp CA BG guys, but then those will still have their aid dist, mob densitiy, aggro range etc set to minimalistic numbers, every door on the map will be removed, there wont be any traps and your payout wont be that good.
What I'm saying is, as every other thing on aop, it's random. And it can be an ass.