That may be true, but as the filter gets loaded with crud (for lack of a better word), the pressure differential between one side of the media and the other side gets more defined. When it reaches a pre-determined point (controlled by a spring loaded valve), the pressure bypass opens and the oil ceases to be filtered. It is just returned to the lubrication system.
I have a post filter on my well water system in the house. As the filter gets loaded with debris, the flow decreases and at a point the flow becomes to low to be of practical use, but as the flow decreases, the clarity of the water becomes better.