What's the water/grain ration you're using for your strike volume? If that's higher, then it'll lower your required sparge volume. There may also be some differences with beersmith settings (i.e., boil-off rate, MLT loss, chiller loss, etc.) but it shouldn't cause 2 gallons difference.
FYI - whenever I brew a new recipe for the first time, the only things I look at are grain bill, hop schedule, and the mash temp. I let Beersmith do the rest of the calculations based on my system and preferences.