Amazing MPG gains... Magic fuel, or ???

I've made several cross country trips and have noticed what Mark is talking about. I generally drive with the avg mpg display active (which is surprisingly accurate in my Chevy). I've driven multiple tanks all averaging around 12-13 mpg then fillup and get 14+ on the next tank. Then drop right back to 12-13 after that tank. Likewise, I've filled up and can barely squeak 12 out of the next tank. I can usually tell within 5-10 miles after fueling if it's going to be a good, bad or average tank. There are the times when I pull out into an obvious wind or have an extended period on a long gradual hill, but other times conditions haven't changed other than the fuel in the tank. Unfortunately I haven't made the trips frequent enough to identify the particular stations, or states, where the superfuel is likely to be found. I suspect it's just a case of blending but it would be nice to build a database of which stations to stop at, and which to avoid. But as kohldad said, it could just be a matter of freshness so the station that gave you good fuel last time you passed this way, may not this time.