It could be a number of things. You might have boiled off more than you thought, or you might have taken the hydrometer reading before the wort was mixed well enough with your top-off water. Usually though, the latter results in lower OG, due to the density of the water. Beersmith estimated your OG at 1.039 with 4.5 lbs of dme. I would assume your boil-off was the culprit.