There are three major systems used to estimate IBU's:
Tinseth
Garetz
and Rager
If the online version uses a different system than you used in BeerSmith, then it won't be the same. Look at the lower right of your design tab and see if any of them are close to 11.9 IBU's. If one of them is, it's likely that the author used that system for estimating IBU's.
Another possibility is that your Alpha Acid % is differen that what the online designer used for calculating IBU's.
It's also possible that both are occurring.