Well, now I have just run across something very interesting........
I am in the process of converting an old recipe into BeerSmith and running in to original gravity discrepancy issues. The old recipe is extract with specialty grains. The OG should be 1.068 but when I build the recipe in BeerSmith, using the extract profile, it comes up with 1.057. Interestingly, when I change the profile to partial mash it comes up with 1.068.
Hypothesis: When you are steeping specialty grains you are actually mashing them to some extent. Thinking about it, the grain is crushed just as in a normal mash; the grain is steeped (mashed) at 154 degrees; the grain is rinsed (sparged) with warm water. This is kinda like a single infusion mash. There is actually quite a bit of sugar that comes off these grains, then extract is added to bring it to bring up the gravity.
Conclusion: What we think of as extract brewing (when steeping specialty grains) should be entered into BeerSmith as a partial mash recipe.
Cheers!