The 72 hrs is the timeframe of which a test must be submitted and result returned in order to meet most travel guidelines.   (PCR test within 72 hrs of international border crossing, etc.)  Ofcourse this will extend to many other industries as well, not just travel.

Essentially, the guidance is for 24-48hrs reply, once the sample has been received at the lab.   With overnight shipping of the sample(say 24 hrs), you get your 72hr window.