So, if Noorden's calculations are correct and precisely sixteen percent of the people are hit by the mists - to the man, he said - that means that not a single one of Elend's fifty-thousand men lied about already being immune.
It also means that none of them were partially exposed, say, by an arm or a leg poking out of a tent that might've immunized them when nobody was paying attention. They might not have even known whether they were immune or not, you know?
I know this is fantasy, but suspension of disbelief only goes so far. I mean come on, at least one dude in fifty thousand must have lied, for one silly reason or another. These are people, after all, and people lie all the time.