Marin
In addition to the closer to a refinery the higher the retail price of fuels, as you mention (it is the same here with Chevron)... currently per barrel cost was last this low in Oct 2011 and it has been decreasing for weeks, yet gas and diesel retail prices keep going up, up, up. At least in this area - they are!
It's no different here. At Costco today regular was (IIRC) $4.11 a gallon. Name brand gas stations are always at least $0.30 higher per gallon than Costco and sometimes even more. Just last week Costco was about $3.90 a gallon (regular).
But having a quarter of the state's refining capabilities down for overhaul has provided an excuse for our price increases and it's expected to go higher as this particular refinery depletes its stockpile of refined fuel, if it hasn't already.