But doesn't it mean there is a lesser chance of having diacetyl?
Ok let me try to explain it. The process for making the 'diacetyl substitutes' involves purification from a chemical that contains diacetyl. So in this instance diacetyl is considered an impurity. I don't have specifics on the percentage of diacetyl is present, all I know is that some flavors that are 'diacetyl free' were tested and found to have some diacetyl still in them.
Percentages of an impurity, or percentages of anything in a chemical term are not like the 'odds at a horse track'. Although it seems to be the same to say there is a 0.1% impurity and you have a 0.1% chance of winning a contest, they are two completely different things. In the case of impurities in a chemical, that impurity
IS THERE it's not saying that 1 in 1000 bottles of this liquid will diacetyl they all have a small percentage of diacetyl.
If I can make another comparison, if drinking water is found to have a level of lead at 3-ppb it means every time you pour a glass of water there is some lead in it. Not that if you pour a billion glasses of water you will get 999,997 glasses of water and 3 glasses of pure lead.
Without knowing the details, there may be a way to make some of these diacetyl free substitutes to a purity level where there is no diacetyl. Until flavor vendors can begin testing these flavors on a batch by batch basis and publishing these tests we don't know if there is a risk to using them.