At some level, it is very simple: you as the traveler are not trusted. Rather, PreCheck makes a determination about each and every trip you take and whether that trip is trusted. At this point, international trips are automatically not trusted, so no expedited security there. A domestic non-stop from LAX to JFK? You won't know until you get to the airport, but it will depend on a variety of risk factors -- the origin and destination airports, other passengers on your flight, your travel status (that is, GE, elite with one airline), and a few other factors.

As to why it's this complex, TSA feels that a true trusted traveler program was prone to penetration and compromise. What they overlooked is that risk (which is vanishingly small) could be guarded against by randomly selecting trusted travelers for additional security. In other words, PreCheck does not have to be this complicated.