you use the same station, same pump, and let it auto kick off with out topping off?
going from 250 miles and putting in 11 gallons vs 250 and putting in 12 gallons is a difference between 22.72 and 20.8 basically a 2 MPG drop because you put a little more or less fuel on each fill up.
If you saw that much of a change in your fuel economy from going from a 89 to a 93 octane (my altitude I live has offset octane levels, 89 is still a mid grade right? what were you getting on 87?) then something is wrong. the motor it self does not run any differently as it can not tell the octane nor is it tuned to advance timing to make use of higher octane. So the only reason why you would see better fuel economy is that the octane is "fixing" another problem.
What could cause this? I am not sure. Another thing to note is that most fuels have detergents in them, some more than others. you can typical get better fuel economy from Costco gas because they typically have less additives like the Chevron or Shell gas.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)