15.0 mpg to 21.6 mpg!

Because when a driver lightly pushes the pedal, and holds it there, to gently accelerate, the engine consumes the limited amount of the shot.
Kindof.
The idle circuit can supply all the fuel needed if the throttle is opened lightly enough. The fuel through the idle circuit also is for off-idle. In the simplest terms idle circuit supplies more fuel to match the additional air because as the throttle opens, more transition slot is exposed to manifold vacuum. The transistion slot is a variable jet (and also variable air bleed).

Accelerator pump shot is needed to briefly cover the delay because the idle circuit can't respond quick enough to the throttle opening. A little more to it (Motorsports Village • View topic - squirter?) but thats the basic idea.