Dry Hopping and IBU's

I did a search, but not finding an answer.
I notice in Beersmith, when you add a dry-hop step to the recipe, the IBU's don't change.
I have to believe that if hops are in the secondary for 14 days or so, some of the Alpha's will get into the beer.
What am I missing?

Hi,
  Alpha acids that add bittering are released during the boil process.  Dry hopping does add other aromatic oils but does not add bitterness to the beer.

Brad