Doesn't it suck up only as much as engine speed and throttle position allows?
No, throttle position and engine load determine the quantity of air consumed.
It can be quite hard to understand at first how a naturally-aspirated engine can ingest different quantities of air at the same RPM.
Here's what the Engine Management Fundamentals chapter of the Bosch Fuel Injection & Engine Management book has to say on the topic:
Fuel delivery requirements depend more than anything else on how much work you are asking the engine to do - or how much of a "load" you are placing on it. To accelerate, you step down harder on the accelerator. This opens the throttle valve, increasing manifold pressure. The greater pressure difference between the manifold and the cylinders increases intake air flow, and therefore fuel flow, to increase power and accelerate the car.
Driving down a level road, you can cruise along comfortably and maintain a desired speed with a relatively small throttle opening. When you come to a hill, it is necessary to pressure farther down on the accelerator to maintain the same speed, even though engine rpm is unchanged. The hill has demanded more work from the engine - created a higher load - and the engine has demanded more air and fuel to match that load.
Regardless of engine speed, the air flow and fuel delivery demands of the engine depend on the load being placed upon it. That load, and the resulting throttle opening, directly affect manifold pressure. Manifold pressure in turn affects air flow and thus fuel requirements.
The above-quoted should be enough to answer your question, but here are my original musings on the topic:
The internal combustion engine is a volumetric device
In other words, it operates by taking in a certain volume of air-fuel mixture during the intake stroke. This is important to keep in mind because...
Volumetric efficiency impacts how much air and fuel is actually taken in
So a 2.0 L 4-stroke engine could be running at 2000 RPM, and one might expect that the engine should consume 2.0 * 2000 / 2 = 2000 L of air-fuel mixture per minute, when in reality it consumes something closer to 1700 L. The reason for this is volumetric efficiency, which is the ratio of what is actually consumed to the theoretical consumption based on engine size and speed alone.
Volumetric efficiency is affected by load
Let's build on the car in neutral vs uphill example by adding a third scenario where the car is running on level ground. The torque required by the engine to maintain a certain speed will vary depending on the external loads on the vehicle as per the ASCII diagram:
-------------------------- TORQUE ------------------------>
Neutral Level Ground Uphill
[2000 RPM] [2000 RPM] [2000 RPM]
Auxiliaries Auxiliaries Auxiliaries
+ Aero Drag + Aero Drag
+ Drivetrain + Drivetrain
+ Car Weight
Different torque ("load") demands will result in the engine altering the volumetric efficiency to adjust the air-fuel mixture accordingly.
So the load on the engine governs the volume of air/fuel that is ingested. This is also why it is possible to determine the absolute load on the engine if one knows the volumetric air flow and engine speed.