When I first starting using Beersmith, my post boil didn't match either, but I changed the settings to my realistic evaporation rate. I don't boil off much, maybe .5 gallon, since I brew inside.
It could be that your preboil efficiency is off because of your efficiency, but the post boil could be off due to evaporation rate, stuff left behind in the kettle, etc.
Broken Leg Brewery
Giving beer a leg to stand on since 2006