During a run, your feet are asked to withstand the repetitive task of shock absorption. A person who overpronates is repeatedly putting excess torque on the medial, or inner part of the leg. Shin splints located on the medial part of the leg and medial knee pain are common injuries associated with overpronation. Just because you sprain an ankle by rolling it outwards on a trail does not mean you're a supinator. They pronate just enough to absorb the shock, and not too much to cause strain on the lower legs and knees.
High arches are usually neutral or underpronators.
Conversely, most individuals with flat arches tend to overpronate. However, after working in a specialty running store for several years, I have seen more exceptions than I care to mention here. The best way to determine your individual foot biomechanics is to undergo a more thorough test. This involves another person such as your coach or running partner. Of course, most specialty running stores are staffed with people who can give you an experienced eye when it comes to determining your individual foot structure.