Many variables will affect your OG and FG (temperature, brewing technique, etc.) This is why it is recommended to check your hydrometer a few times. Once the gravity is no longer dropping, fermentation is done. In your case, you didn't hit your original gravity (maybe too much water or some other minor mishap) and then I would guess your fermentation conditions were less than idea so your yeast did not fully attenuate (ferment to their full potential). Keep up the good work, and I am sure your beer will be just fine, there is always room for improvement, that why this is such a great hobby! Keep, reading on here and try to make your brewing techniques better and your beer will continue to improve.