Breathalyzer devices are not always accurate. A lot of different factors can throw off the accuracy of these machines, like the calibration of the device, how long it's been since the subject drank and the way the test is administered. Another thing that can interfere with breath test accuracy is "mouth alcohol."
Mouth alcohol happens when alcohol gets inside the mouth itself, creating additional alcohol fumes that wouldn't otherwise be in the breath. This can create a false reading that doesn't accurately reflect a driver's actual blood alcohol concentration (BAC).