It was basically because the SC (NB - first time a SC had ever been used in a race) was driven by a Joseph Ellis wannabe, and picked up completely the wrong driver when he came out after Scheckter and Cevert crashed into each other. At that point half the field had pitted for slicks (it had been raining), and so the other half pitted straight away. After all that, and the safety car came back into the pits, no-one actually knew who was winning (no electronic timing or anything), and so the organisers had to guess what happenned during the SC period as to who won. The wiki article displays the official results, which was that Revson pitted at the exact right moment and gained an entire lap under the SC, followed by Fittipaldi and Oliver