You're allowed a certain level of THC however, if you're over the limit then you can test again 48 hours later. If your level goes down then you move on to the next week or so. If it remains the same or increases, you've technically failed the test.
Athletes have figured it out - test high, don't use it for 48 hours and your level will drop. Boom, you're cleared to play that weekend. The two guys in the baseball program chose not to take this route and it ended up out of the hands of the coaches. When Jelkin got busted, 7 guys failed that test but he was the only dumb enough to continue using it, failed 2 days later & UNL booted him for failing too many of them.