One unfortunate reality of law enforcement is that police officers have flaws. They are human. They may even make serious mistakes and issue false arrests for a variety of reasons, some malicious.
For example, perhaps one police officer does not like how a driver responded to a question after he pulled him over, and the officer decides to charge him with DUI. Another officer might target racial minorities by inventing reasons to pull drivers over. False arrests are phenomena that occur across the country.
Suppose a police officer pulls you over and asks a question. You answer to the best of your ability, but something in your tone, word choice or expression angers the officer. The officer might then falsely arrest you. As a result, you could face imprisonment, fines, driver's license suspension and hikes in your insurance bill.
It happens sometimes that a driver passes a chemical test (or refuses to take one), and there is no real evidence to show he or she is under the influence. Still, police officers have a "gut" feeling, or suspicion, that the person is legally intoxicated, and make an arrest.
Some police departments have DUI arrest quotas. These can spur police officers to make false arrests rather than put their jobs at risk. Alternatively, a police officer may bask in the accolades he or she gets for having a high number of arrests and keeps making them.
No reason is perhaps more grievous than that of bias/prejudice. That is, a police officer falsely arrests someone for DUI just because the officer does not like his or her skin color or ethnicity. The officer may have pulled the person over for a legitimate reason or could have manufactured or invented one. The driver could be perfectly sober, having had nothing to drink for days, and still face arrest.
Of course, it is possible for a chemical test to malfunction and a false arrest made with the police officer believing in the test. However, if a police officer arrested you while you were sober, one of the above reasons may apply.