Why does the air pass through the intake sensor first and then the throttle? Won't this cause the air-fuel ratio to be out of proportion as shown in the figure?

There are two types of air intake sensors, that is, before or after the throttle. Generally speaking, the air-fuel ratio of 14.7/ 1 can't be wrong, but when the vacuum tube leaks, the idle speed behind the throttle valve is high (vacuum pressure leaks, and vacuum reduces fuel injection).

The measured signal before the throttle is small, the fuel injection is small and the idle speed is low.