Every extract kit that I have brewed mentions to top off the primary fermentor to the 5 gallon mark shortly before you add the yeast. It gets you the correct amount of beer and also helps cool it down to pitching temp. Is there a chance that you just forgot to top it off when you were supposed to?
Its step 9 on the directions sheet.
I am about to share my La Petite Orange with the family in about 20 minutes myself. It turned out wonderful. Reminds me so much of a Weihenstephaner Hefeweissbier even though its not a hefe. Such a good beer either way tho