Might be because it's changeover time for gas. Time to change the formula from "winter gas" to "summer gas" So, if the refinery is shut down or on a slow down, they'll be producing less, thereby keeping the demand for available gas up. This would keep the gas prices up. If diesel is not being affected by a "slow down" then there would be more available and the price would be down, or at least not as high.
Seems like "old times" when gas was always higher than diesel, but I'd guess those years are waaaaay behind us and gas will fall enough to keep diesel up there.
Funny how when the speculators suggest oil will rise in a few days, fuel prices go up, but when the price of oil actually goes down, fuel prices don't follow as quickly on the downslide as they do on the uphill side..... Hmmmmm oil companies just keep working their profits any way they can