The airlines can sell more seats than there actually are on the airplane. It is VERY common for passengers to miss a flight, so they overbook the flights and make some extra money. The capacity is what the airplane actually holds in the back (no jumpsuits) and the AU is how many tickets they are allowed to sell to paying passengers.

It doesn't have anything to do with non-revs or jumpseating. You can list non-rev for an oversold flight, but it just means that there is less of a chance of getting on that flight.