The BAC reading from a breathalyzer always has some error associated with it. The more expensive breathalyzers, especially those used by law enforcement, have less error than consumer breathalyzers, but no breathalyzer is 100% accurate.
Your blood alcohol level as measured by a breathalyzer is in the units of % BAC. So a BAC of 0.080% (the legal limit) means that 0.080% of your blood by weight is ethanol. A professional breathalyzer must pass testing done by the Department of Transportation, which requires an accuracy of plus or minus 0.005% BAC at a level of 0.100% BAC. The FDA requires that a personal breathalyzer undergo accuracy testing too, but the accuracy requirements are much less stringent: the device only has to verify whether there is alcohol present or not, but not with any real accuracy. Most breathalyzers that have been granted FDA certification state their accuracy to be plus or minus 0.010% BAC at a level of 0.020%. This would be equivalent to a professional breathalyzer being accurate to a level of plus or minus 0.050% at a level of 0.100%! Quite a difference.