I remember watching an anti-alcohol video in high school wherein they gave 4 or 5 people one drink after another and had them drive an obstacle course increasingly intoxicated.

All but one of them got worse with additional alcohol, but the last guy, who was probably a high-functioning alcoholic who drank daily, continued to drive faster and more precisely with each additional drink well past the legal limit until his skills collapsed catastrophically at around twice the legal limit.

So basically, I suspect it's hard to come up with a uniform standard here so they err excessively on the side of caution and use a limit where an otherwise teetotaler would be impaired. Finding out not even that is accurate is not encouraging.