In the short run, it isn't. The distinction between short and long run aggregate supply is that people react differently to the same events, depending on whether or not the events come as a surprise. Thus, in the short run, people are surprised by price changes and so they change their output decisions more than they would if they had anticipated the change. In the long run, people are not surprised, so they do not change output in response to changes in the price level at all. A long run equilibrium of Aggregate Supply and Aggregate Demand is that same sort of plan-fulfillment equilibrium.
People make their plans and things come out as they expect, without surprises.