There's a number of reasons a drug test might be "inconclusive" - they measure a lot of things besides just if there are drugs in the urine. The temperature has to be right, it has to appear not to be diluted or altered in any way, which is determined through various chemical analysis techniques. It could very well have been someone messing up at the testing facility, like if they didn't take the temperature fast enough or they didn't collect a large enough sample or something. Different labs have different policies, so you would need to ask them for clarification on what specifically made your test inconclusive instead of positive/negative.