Having been Plod before flying what you have to take into account is that a breath testing device used on an aircraft or at the roadside (Its the same one, you just press a different button to set the limit) is a screening device only. It gives a fairly accurate indication, that gives the required grounds needed for an arrest. It DOES NOT constitue evidence for prosecution. On arrest a person is taking to a Police Station where they are put on a calibrated breath test machine. If they then provide a positive sample then it is this that is used for a charge. If it is negative, as would appear in this case, and impairment is still suspected then a blood sample is taken, and analysed by a lab for a result.
I would say in my career about 30% of the people who provided positive breath tests at the roadside gave negative results at the station.