Just a quick note regarding Seattle. As everyone knows, the A's are the first 45 people to check in. The B's are the next 45 and the C's are everyone else. There are overhead signs showing where each group should line up. There are also stanchions that have divider belts. Granted those belts and stanchions don't go very far back and thus, don't create shoots. When flights aren't very full and there are perhaps only 3-5 B's, the Ops agent will just announce that he/she will board everyone as A's. Perhaps that's what happened on the day you were flying out of Seattle.