Cynicism, the test relies more on changes in the conductivity of the skin, which changes when your autonomic nervous system is being changed by stress and emotions.
The lie detector test is based on heart rate. Of course they have to do a base test where you say known facts about yourself so they can determine when your heart rate is going wonky. If he lies during the base questions his other lies don't show up as such.
Of course if people are nervous they can also screw up the lie detector test, which is why they are not admissible in court.