If it's extract, the answer is that you didn't mix it well enough. If you used the specified amount of extract and hit the correct volume of beer, you literally cannot end up with any other SG number - there is a set amount of sugar available in that extract, and it's now all in the beer. Thing is, it's tough to get a complete mix of heavy wort and plain water. Don't worry, though - the yeast will do it for you.
This is a ridiculously common newbie "mistake" that's really a nonissue. Many, many, many new brewers "miss" their OG, when it's just a mixing issue (I did it a couple of times before I learned). Just use your kit/recipe's target OG and go with it.