IBU's don't match up

I bought a CDA kit (extract with steeping grains) from homebrewstuff and decided to plug it into iBrewmaster to see how the stats measured up with the sheet they sent along in the box. ABV, SRM and gravity matched up pretty closely, but while the recipe had the IBU's at 81, my software gave me 97.5. 20 points off is enough to make the difference between good and drinkable, so I'm wondering what the deal is. I just got iBrewmaster a couple weeks back and this is my second brew with it. Any suggestions or ideas?

If you are doing partial boils you will loose some hop utilization due to the density of the wort, vs a full boil.

could have used a different formula and with it being an extract full or partial boil would matter

I do full boil. Would the recipe from homebrewstuff assume that I'm doing partial? I haven't seen anything in iBrewmaster that can calculate for partial boil yet, so I'm assuming that it's assuming full boil. That would account for it. I'll reduce the bittering hops by 30% and see what happens.

There are various models that attempt to predict the IBUs, ie Rager, Tinseth. They may have used a different model with slightly different inputs, but in the end it doesn't really matter. They are just models. You'd have to get the finished product to a lab to know anyway... And it's often shockingly off at higher IBUs.

We can only perceive so much bitterness (I've seen 80-100 as the limit). After that we are perceiving flavors and aromas from the extra hops, but our taste buds are on bitter-overload. I would just go with it and not worry about the numbers. If you use the same ingredients as the recipe, you are brewing the beer that was intended in the first place.

the AAU % on the hops in the kit may be diferent from what you have set on !brewmaster. You should be able to change waht is on !brewmaster to match what is on the package that the hops came in.