Have you ever ran a model engine before? Its a matter is physics.. The exhaust pressure is more effective at pushing fuel to the carburetor when the tank is full, as the level drops, the airspace inside the tank increases and pressure acting on the fuel drops thus leaning the fuel mixture. A 3-line Uniflow tank setup averts this to a point, but you end up with fuel getting pushed out the exhaust through the pressure line if you drop the throttle abruptly.

If the fuel system was regulated by something other than the carburetor, the low tank lean-out wouldn't be as prevalent.

I'm sure others will have a different explanation than I gave, but the principle is still the same. Full tank = rich mixture, Empty tank = lean mixture. Its the reason engines have to be set rich at a full tank. By the bottom of the tank the engine is running at peak or slightly richer than peak rpm.