That is how they work. The placement of the hole determines at what point in the throttle position it is opened or closed is going the other way. The air bleed does effect the fuel mixture at other positions other than just idle. It effects the mixture at idle and off idle until the barrel closes the port.

Now one of the benefits of an auto mixture carb is that the fuel orifice can/does control the fuel much better at throttle position other than full power (more on this latter).
Because of this superior ability to control the fuel flow the engine designer can make an engine with great flow capacity, both in port area and timing. The air bleed carb is limited in its ability to deal with any radical flow reversals at low speeds (idle). Now because the auto mixture carb can deal with more radical air flows at idle it affords the designer an opportunity to give us engines that a more powerful yet track/handle better. While it is true that for any given choke are both carbs will allow the engine to perform the same at full throttle. The engine and carb need to be looked at as a system. With the auto mixture carb the engine designer can give use a more powerful engine that actually idles better ,is more flexible and has more power. The true air bleed today is only on the market as a cost saving device! Duke has always avoided the true air bleed carb in an effort to supply us with only the best!

Hey, it is a pro Fox thread after all!

All the best,
Konrad