I must have a lead foot, I always seem to get just around 20-22mpg with my Duratec. I also always run regular 87 octane. How many times have you filled up the tank? I've noticed that my gas milage does seem to fluctuate, I think it has something to with the pump I'm using, some seem to shut off sooner than others because sometimes I'll be able to drive for 30-40 miles without the need moving and sometimes after only 10-20 miles the needle will drop so you might just have to average out your gas milage over several more tanks to say that you're really getting 23 instead of 27. Of course with that in mind, you might not have really been getting 27 all this time.
The other thing is that octane is the ability to resist knock. The btu content of 87 octane gas is the same as 93 octane. If your car has lots of milage and has hot spots in it such that you're getting knock, some of the power is being wasted in the knocking and using a higher octane gas will eliminate that knock and deliver more power. However if it's not knocking, then you're just wasting money by using a higher octane that the car doesn't need.