Consumer Reports reported on the Fit and other budget cars in their 12/2006 issue. Their tests of the Fit showed city MPG of 22 for the automatic, 26 for the manual; overall 32 and 34. You don’t say which trans yours has, but your 21 and 23 are pretty close to CR’s 22 for the automatic.
The comments above about driving in winter temperatures and using gas that has ethanol in it ring true.