Lug set-back involves excess pressures hammering the bolt back into the recesses which creates a concavity in the mating surfaces of the action. Generally there is a small bump behind the bolt lug due to the upset of metal.
The brass in the chamber has been fireformed in the chamber and therefore is a little bit bigger than when inserted (yes there is some spring-back in the brass, but it is still bigger) and the combination of the little bump behind the concavity and the bigger brass makes the bolt difficult to lift. It will also be a little tight when closing on that fired brass.
On Mausers small incipient cracks can often be seen radiating forward from the rear back corners of the bolt lug after set-back has occurred.
