It may also be about different qualities. Light crude is easier to refine, so when refinery capacity is limited, demand for light crude increases. The average/reference price (light/heavy mix) then also rises. When there is plenty of spare refinery capacity, heavy oil is (almost) as good.

If this theory is correct, it should be possible to discern a refinery capacity crunch by the divergence of light/heavy crude price. I don't know if that is the case now, and five minutes of google didn't tell me anything.