There's a practical reason for that...Beauregard wanted Sumter in good shape when they took over. Does that perhaps presuppose that he knew Anderson would surrender after saving a little face?
BTW; my image of Scott comes from the Errol Flynn movie, where he played Custer. "Boots and Saddles"..."They Died With Their Boots On"? Sydney Greenstreet played Scott. You might remember the scene in the Willard Hotel dining room, where Scott continually ate boiled onions as he talked to Custer. Stan