The reason is because the pump box is an open system. The pump intake is one inch and is pump driven (high velocity). The vertical pipe is also one inch but is gravity driven (low velocity).
The difference in volume is made up by drawing in air through the top of the pump box. You could seal it (not very practical but do-able), you could remove the sponge filter and plumb it through, or start cutting holes
I don't remember the math, but I think the ball park result was four one inch holes or a single two incher. (The correct answer is velocity dependent.) Btw, the plastic strainer on the bottom of the vertical pipe puts restriction on the pipe and reduces it's flow.
The difference in volume is made up by drawing in air through the top of the pump box. You could seal it (not very practical but do-able), you could remove the sponge filter and plumb it through, or start cutting holes

I don't remember the math, but I think the ball park result was four one inch holes or a single two incher. (The correct answer is velocity dependent.) Btw, the plastic strainer on the bottom of the vertical pipe puts restriction on the pipe and reduces it's flow.

