2 1/2 hours is a fair amount of time between tests. It is possible that the first one was in the ball park, but the delay between the two tests bought the reading within limits, whatever that is. This sort of thing happens often with alcohol tests of drivers. The second test is a requirement I think, and if the calibrated machine is a fair distance away, then you end up with the delays. Useful tip if anyone finds themselves in this situation. Waste as much time as possible getting to the second test.
Before all those who have never sinned jump on my back, I agree that none of us professionals should ever end up in this situation, I'm just sparing a thought for those among us who occasionally miscalculate.