That's because THC is fat solvable and it stays in your system even when you aren't high. So even people who haven't smoked, but are using bodyfat for fuel. As you burn fat, your body can release THC.
I have a friend who hadn't smoked in more than 24 hours and his blood test came and he was above the legal limit because he us a heavy smoker and even when not high, his blood work is higher than rare smokers who are stoned.