First, onboard computers are not good for getting an accurate MPG rating. Go by what you actually fill the tank with and how many miles you drove. The scangauge will also give a better reading than the onboard computer. Also, I don't know how long you've had it, but you should be comparing your MPG tank to tank, not day to day, because there are a lot of factors that can affect it.
Here's the EPA's website, click on 2001 Jeeps-
Search by Manufacturer
Depending on your model, you're looking at 13-14 city, 19-20 highway. Maybe you were going downhill with the wind or something for the 26-27MPG that you saw.
If you bought a Jeep Grand Cherokee expecting good gas mileage, then you made a foolish purchase.