A filter is only as good as it's seal!
You can have the best filter available, but if it is not sealed in the housing correctly it will bypass and essentially be worthless. The photos below dramatically illustrate this. Notice the distinct color difference at the end of the filter cartridge. It is very clear that the knife edge of the sealing seat cup is biting into the filter and creates a perfect seal. If you suspect your filter is not performing properly then take a closer look at the end caps of the filter cartridge next time you change them. You should see a distinct line of separation between the dirty outside diameter and the clean inside diameter. If you have actual plastic or plastisol end caps you will not see the dirt separation. You should however see an indentation formed by the knife edge seal seat cup.