Well, the one time That happened to me with a Fram, I cut the filter open and found the pressure was low because the filter had collapsed internally.
But most people (including me) do NOT have problems with "low oil pressure" whn using frams. What we've had problems with is NO oil pressure on start-up because the anti-drainback valve didn't work, and allowed the filter to fill with air overnight, so that the pump has to run for several seconds to blow the air out (through the bearings, I might add, which disrupts the oil film already present on the bearings) before pressure will build.
And if you think (as you imply) that other brands leave a lot of junk in the engine that the Fram mysteriously is able to remove (despite having less surface area and a filter media with the same pore size) then I challenge YOU to cut one open and prove to me that it is indeed "heavy" because it has trapped a lot of stuff out of the engine. I also find that hard to believe because none of my engines have ever shown any sign of any sort of internal deposits when I've pulled oil pans or valve covers to replace gaskets.