Random drug test

How random are the drug test at usic? I started may 11th and just certified today and had to quit because I of course got picked out of like 200+ people for a drug screen. I knew I wasn’t going to pass and luckily my sup was a cool guy and understood, we live in a legal state so of course of smoking way before I got this job but when I figured out there was random test I stopped. He said he would make it a self resigning, and had a family emergency so I wouldn’t lose my license, but a part of me just doesn’t feel like I was just the unlucky guy and kinda feel like of course the day I certified I get tested. He said in 6months we can try again. I just honestly feel very stupid and genuinely loved working for usic and definitely wanna try again, but just wanna know what are the odds? Thanks in advance.