God. This is gonna prove I have no life. I wondered the same thing several years back. So, I took a new pad and an used but rinsed out pad and compared them under a dissection microscope. I found that even though the used pad's fibers were clean the fibers were flatter and the weave was tighter than the new pad's. As the pad's fibers and weave lose resiliency, they lay closer together and cause more of a blockage against water flow.