Law enforcement officers have some discretion when pulling you over for a suspected DWI. There is no clear way for them to know if you are under the influence, so they judge the situation based on how you are driving, looking for common signs of intoxication.
According to the American Addiction Centers, the only requirement to pull you over is to have a suspicion that you are operating the vehicle while under the influence of a substance. Anything the officer sees can be a basis for pulling you over.
The next step after pulling you over is to put you through some tests to determine your sobriety. The traditional field sobriety tests involve several movements and activities meant to allow the officer to gauge whether you are under the influence.
However, the best way to test that comes without any opinion of the officer and has a basis in science is a blood alcohol test. On the side of the road, you will typically take a breath test, but an officer may also place you under arrest and test you at a later time through blood or urine tests.
Despite the testing, the officer can still place you under arrest for suspected DWI just based on his or her opinion of your behavior and actions. If the officer thinks there is any impairment, then that gives him or her justification to take you in for a DWI. Largely, this subjectivity is because there are some substances which the tests cannot detect, such as marijuana.
Giving officers the option to use their own discretion means you could go to jail even if you are 100% sober.