Since you are brewing with extract, I would bet that your OG was a bit off from your overall volume measurement. You probably had a bit more water in your fermenter (or kettle) than the recipe in beersmith was counting on, giving you a slightly lower OG.
The other thing that can happen is that the brewing programs (beersmith, promash, etc.) can sometimes have default gravity potentials (PPG) for ingredients - extract or grain - that are higher or lower than what the ingredient actually delivers. By gravity potential I mean the number of possible gravity points per pound of the ingredient per gallon of water - PPG. For extracts, you automatically get 100% of the potential gravity points. For grain, you get a percentage of the potential which is your system efficiency. These default values in the software can be off a bit - even for extracts. According to Promash, values for LME range from 35 to 38 PPG depending on the brand. This can definitely affect your OG.