Jialing Motorcycle 125 thimble machine, when adjusting the valve, the end point is in the small hole on the left cover of the machine. Why is f not t? What happened?

After point F is point T, the piston will stop briefly when it reaches the top dead center, and then carefully observe whether the piston is at the top dead center when it is aligned with point T. Point F is the ignition advance angle, and the timing of point F is slightly earlier than the compression top dead center because it needs to be ignited in advance just before the piston reaches the top dead center.