Partially because the national gasoline supply reserves (actually held regionally) are being purposely drawn down. Do not confuse this with the strategic petroleum reserves. The gas reserves are fully processed and ready-to-use gasoline. The gas storage is being essentially eliminated for a few reasons ...
- it's a dangerous commodity to store in large volumes (both environmental and terror target concerns)
- it degrades quickly in its finished form (it has to be used and replaced often)
- our nation now has a very large capacity for processing raw products into fuels quickly (the highest volume in the world)
So, as these regional volumes are drawn down, the "supply" is a bit more abundant than "normal". Once gone, those supply/demand curves will adjust again.
I'm not saying this is the only reason for retail prices moving up/down, but it is a contributor.
Obviously, pricing goes regionally due to a slew of reasons.