With extract brewing, its nearly impossible to not hit your target gravity, unless your volume measurements are off. So, one thing to check would be if your recipe was for 5 gallons and you have five gallons of wort fermenting away.
The more likely culprit, though, is that your hydrometer reading was off, because the wort from your boil and the topping-off water didn't get completely mixed, which is very common. So you were taking a reading from less dense wort.
I suspect your original gravity is fine, just not measured correctly. Cheers!