If the road is usually congested, frequent traversers of that road will just get in the habit of staying left until they are near their destination. I do it on PA-22 all the time (albeit not in a condition where people need to pass me on the right), even when the traffic is light, because its what people expect behavior wise on that road.
By that logic, that would mean at 11pm at night on US-101 between SF and San Jose, I shouldn't need to pass anyone on the right, but I guarantee you there's always drivers lingering in the left-most lane.