No at-home drug test is 100% accurate. Many factors could alter the results of an at-home test resulting in a false positive or false negative result regardless of the person's actual drug abuse.
For instance, it is possible to test for the wrong drugs. Timing of the collection could be off and the drugs had not yet shown up in the urine at the time the sample was collected. It is also possible that the chemicals in the at-home test went bad if they were stored incorrectly or they passed their expiration date.