Beersmith 2 IBU calc

Hi
I'm having some trouble with IBUs in Beersmith 2.

My hop schedule for 10L is
15g Centennial 60min
7.5g Centennial 15min
7.5g Simcoe 15min
7.5g Centennial 5min
7.5g Simcoe 5min
with Simcoe at 14.1% and Centennial at 8.2%

I'm getting 87.4 IBUs. While if I use the "Hop Bitterness" tool I get 53.8, which is similar to what I get using other online calculators.
Based on what I can find in the forums I gather it's probably something to do with my equipment setup but can't figure out exactly what.
File is attached. Would appreciate any help!

Cheers

I looked at this when you first posted, but not closely enough or with enough time to really explore your recipe.  The difference in IBU values is due to the late addition of DME.  You are boiling for 55 minutes with just water, which according to the utilization formulas will give you a better utilization of the alpha acids.  In reality, I am not so sure of this but have never tried it to see what the result would be.

The reason it differs from the hop bitterness calculators or other on-line calculators is that if you state your beginning gravity as being the gravity with the DME, it will calculate a lower utilization. 

Thanks so much for that, it is the late adding of extract.
I had no idea this would effect the IBUs so much. Even adding half early half late makes a massive change to the IBUs in the software.
Interested to hear what other people do regarding this - just enter all extract as 60min boil even though you may add a large portion of it late?

You can split the extract additions if you so choose.  Remember also that the IBU calculations are an estimation based upon one person's system and process.  The relationship between what the calculations give you for an IBU figure and what you actually achieve may be somewhat different.