I cannot say for sure why a better filter gives better oil pressure, but a couple of possible reasons come to mind. One is that when a filter clogs and bypasses, pressure drops, so obviously a filter that does not clog will work better longer. The better filters have many more pleats, meaning much more working area, than the cheap ones, and thus do not clog so soon.
I suspect there also is simply a different, and better, medium of filtration in some, and that fineness is not the whole story. I have not studied the filter media, though I did cut open a bunch of filters some years ago. Most looked similar, but the media themselves were clearly different, with the best not only having more pleats but what appeared to be a more solid material. The one exception was K&N, whose filter medium was entirely different from the others, harder and less papery.
Whatever the science, when I was putting a lot of miles on my 87 I went through a lot of filters, and analyzed several. I settled on Wix as the best bargain in good filters, and have almost never bought anything else in the half million or so miles that have followed,, except when they could not be found, and I got Bosch or NAPA gold instead.
Who knows what might have occurred otherwise, but I have taken several XJ's well over the 250 thousand mile mark with utterly no internal engine problems, and in the process maybe spent 40 bucks more on Wix filters than I would have on cheaper ones.