He appears to have apssed the evidencial test conducted at the Police station. Be that breath or blood. The obvious reason being that his BAC was going down and went below the limit between the first and evidencial tests.
Quite possible but there is also another reason, and this is from experience.
One evening after a few bevvies in the pub I stupidly got into my car and drove home. In the good old tradition of sods law I was pulled over for exceeding the speed limit (only slighty) for the first time ever.
The resultant breathaliser test proved positive (according to the police officer only just - not sure how they tell this) and I was arrested. At the station I undertook three official tests (which would be used as evidence). The police took the lower of the three tests as my result. It was over the legal limit by a fraction. I cant remember the exact figures sorry. However I was released without charge as the CPS will only prosecute above a certain percentage to allow for erorrs etc.
So although my name was 'clear' I was still breaking the law. What bought it home to me was that I did not feel drunk or even slightly impaired...but who does?
The officer did not get his 'collar' but as I explained to him out side the station he still had acheived his goal as I have never done it ever again and nor will I.