This phenomenon is known as residual mouth alcohol. And it can affect the reliability of an approved screening device (or, roadside breathalyser) test. But it can also affect the reliability of a breathalyser at the police station, even where the instrument has a mechanism designed to eliminate it.
The video above shows how mouth alcohol falsely skews the results. I do not want to spend much time on the process, but more on what I have seen and where mouth alcohol can come from in affecting a breath test.