Wily dogs are those who are aware that their owner cannot see them, and use it to their own advantage. On average they wait longer to take the treat when their owner is looking than when their owner is not looking. Trustworthy dogs do the opposite. They wait longer on average when their owner isn't looking--they know their owner isn't aware, and decide not to use that information against their owner.
Dogs that wait the full 90 second in every condition end up right in the middle of the spectrum as Mixed. Why? These dogs showed the same behavior when their owner could and couldn't see them. Clearly these dogs are spectacularly obedient, which is no easy feat. However, research shows us that obedience can vary widely by context even for the most trained dogs. So in different contexts these dogs may choose different cunning strategies. In some contexts the dog may be trustworthy, in others wily, or the dog may not really be paying attention to what their owner can see!