I agree that the annotation said that the mist would snap people who had lower-than-normal allomantic potential. However, I don't think it JUST snaps those people. I found a place in the annotation where it mentions that it would snap people with normal potential as well:
http://brandonsanderson.com/annotation/345/Mistborn-3-Chapter-Seventy
I suppose the 16/100 would still be better than 17/100 since part of the point was for Preservation to leave the number 16 as a clue. In which case, the 1/16 people who stayed sick longer could have still been mistborn (16 people each with an alloy * 15 groups, 1 group of 16 mistborns).