Okay let me shed some light on this. I have worked in a hospital lab for 20 years (I currently now work as a tech in a Cath lab now) anyway during my tenure in the lab, we would also do drug screens on employees (I have seen it all - we had one guy who came up positive for 8 different narcotics and he wanted to work in a hospital)
Anyhow, people can come up for false positives for a variety of different reasons. The testing gear is very sensitive and can detect the slightest trace of any drug. In which case it will just say "Positive" unless the required company or department requests that further testing be done on urine sample. In your case I can almost gaurantee they did not request further testing to see the percentage totals.
Eating a poppyseed muffin will give you a false positive for heroin, that's how sensitive the testing gear is. Now you said you havent smoked in 6 months and I believe you, but if you have been hanging around others who smoke, that will get inhaled by you (second hand smoke) the gear will pick up on that, if you attended a rock concert and some guy near you was lighting up, you will come up positive. Your best bet is to ask for a second test, accidents happen. You probably got a false positive (which happens more times than you can think of) and because of your past history, your fate was sealed.
Also what the local manager or HR department says will vastly differ from what corporate thinks. Your manager can say your hired, but a copy of that background check gets sent to corporate headquarters and they will ask if you are still hired and if you are, they will ask that you be terminated on the spot. Having never met you or care to know what type or person or worker you are.
As an employee (former or current) you have the right to ask for a retest and you also have the right to ask them to request to retest the sample for a concentrated amount to determine if you were taking bong hits the night before or as I mentioned you happened to walk into a room where somebody else was smoking.
Good luck.