Every barrel of oil imported to Oahu is subjected to additional taxes and fees, in accordance with Hawaii gasoline laws. For the neighboring islands like Maui, Kauai, Big Island, etc. the prices are even higher because of the cost of shipping and the county taxes are higher.
Pricing trends are also different here. There have been times when prices on the Mainland will fall but our prices go up! Or, our prices will start falling a few weeks after Mainland prices fall. I've read Hawaii buys its crude oil from Asia because its closer to ship than the Middle East. Maybe that plays into the disparity as well...