In this video, a former DUI prosecutor explains that DUI charges are rarely cut and dry, and often can be attacked successfully such that the charges get reduced or dismissed.
The district attorney will often present the case in such a way that he thinks the evidence is overwhelming and clearly indicates that the motorist was driving under the influence.
But a good DUI defense lawyer can often dispute each piece of evidence and raise doubt in the minds of the judge and the jury as to whether the client was truly intoxicated at the time of driving.