It probably means they're using a cheap quick test with a relatively high rate of false positives for various reasons. The chemistry is complex, and similar chemicals could be responsible.
Rather than straight out call it a positive, they will ask for a second test. This is more expensive and lots more sensitive and reliable. Overall the company saves money by using the first cheaper test as a filter.
Drugs can stay in your system in trace amounts for quite sometime. Cannabis, for example, is easily detected in urine for up to a month. Other tests can find it in hair samples for much longer.
Simple rule is - if you ever expect a drug test in your line of work - don't smoke or take drugs, ever.
If this is for a new position - well, be prepared to be knocked back if not enough time has passed since you stopped.