GD - its about the pressure waves in the air. With a TB narrowly open, the air is rushing in at a faster speed, so as it starts to pile into the cylinder, the air pressure in the cylinder increases. The air in the inlet manifold and intake pipes still has a lot of forward momentum and carries on flowing towards the cylinder, piling more and more air in, even if the pressure in the cylinder goes above atmospheric, a little bit of boost in a way. If you have a wide open throttle at low revs, there is no speed to the airflow, so no momentum pushing more and more air in. Obviously at high revs, this idea goes way out the window, the throttle body is a restriction pure and simple so even wide open, the engine still can't get enough air.
The only other thing that bothers me, is with a mostly closed throttle body, the air is being sucked in faster, the engine is having to do more work sucking against a narrower aperture, so instead of the piston pulling the air in freely, it is having to do work sucking it in against a restrictive TB. This is the same as the engine losing energy having to push the exhaust gasses out against a restrictive exhaust system.