There are two ideas behind this:
1. Make sure your throttle plate opens fully at WOT. Sometimes, at WOT the throttle plate actually swings past the fully open position and becomes semi-closed, if it's not set correctly.
Have someone sit in the car (with it off), and with the intake taken off, observe where the throttle plate is when the accelerator is a WOT. Adjust if necessary.
2. The problem is at lower rpms an engine needs to ingest air slower than when at higher rpms. As engine speed increases, piston speed increases, and therefore the amount of time that a cylinder has to fill is decreased. At that point you need the air to be moving faster.
Much like an exhaust, where if you go too big (WOT), the air will not flow as efficiently through the intake tract (this includes the intake, manifold, AND throttle body. You effectively change the size of the intake by how far the throttle is depressed. What you're noting is how the throttle position affects intake efficiency.