A possible hypothesis for this phenomenon is that at lower speeds, the filter elements of the air filter, which is made up of a series of wire meshes, could actually be aligning the flow of the air entering the cylinder, creating an area of laminar flow. At the same low-throttle-opening position, the throttle body without an air filter is actually drawing air from all directions, creating a turbulent flow area at the centre of the throttle body opening, reducing the air flow through the throttle body.