I was probably the most surprised when somebody would blow under. Sometimes the symptoms of a less experienced driver blowing only 0.90 and being behind the wheel of a car were quite frightening. So it does vary, based on a person's size and also their tolerance.
I did about 400 tests over 20 years ago when I was a breathalyzer technician. The other big factor for me is that in most cases by the time a person gets to the breathalyzer it's probably one to two hours since they were driving, so there's probably at least another drink in their system, perhaps two, that has burned off during that period of time. They're going to lose about 0.015 every hour from the time they stop drinking.