Corn sugar can also dry out the beer or make it cidery, depending on how much is used.
If you did not do a full boil then chances are your beer wasn't mixed well with the top off water and gave you a false reading. It happens a lot.
For this reason I recommend taking a reading (then give the wort a slow stir or two to meld the wort and water) than take another one about 15 minutes later before adding your yeast. The final measurement will be the correct one.
If you followed the recipe then the sugar you need to make a good brew is already in there as well as the proper amount of hops to keep it balanced. Adding more sugar may throw that balance off and produce a sweeter brew.