There is 1 line to start and it splits before the stairs. You have your choice of going left or right. I usually go left and there may be 1-2 people in front of me max. There is never much of a line in the single rider line.
The confusion there is because the split is on the left. The "right" stairwell is basically straight ahead when you walk down the hallway from the single rider entrance. People see it immediately and walk straight ahead up it, not realizing that the "left" stairwell is behind them. So the "left" stairwell usually has fewer people in it.