BeerSmith question

I just downloaded the trial of BeerSmith and plugged in some of the kit recipes that I've done. I'm finding some big differences in projected IBU's.

The "Liberty Cream Ale" kit from Midwest, according to the website is 33.4 IBU's, yet BeerSmith is telling me it's 12.1 (with a 2 gallon partial boil). Even with a 5 gallon boil Beersmith tells me it'd only be 21.8 IBU.
Here's the recipe:

6 lb. Gold liquid malt extract (60 min)
8 oz. Carapils specialty grains
1 oz Cascade (60 min)
1 oz Cascade (2 min)

So who should I believe?

I've never used Beersmith, but does it have a field for Alpha Acid?

Do the actual alpha acid % match up with those in beersmith? I often find that they vary and you have to change the value in BS.

Yes, the hops are listed as 5.5% alpha acid. Midwest has them at 5.4% so there's not much of a difference there.

I would beleive BeerSmith, but that's just me.

I've always found BeerSmith to be pretty accurate. Not that I've ever measured it, but comparing what it predicts for my recipes with how the end results taste, then comparing that taste with other commercial brews, the results seem to match up.

Do you know what boil volume the original recipe was assuming? That makes a huge difference to the hop utilization. If they were assuming a 5.5 gallon boil, for instance, that would explain the discrepancy. Late extract addition (where you save 2/3 of the malt extract until the end of the boil) also has a big impact here: I do that with partial boils to get an end result closer to a full boil.

I would check all your setting in Beersmith to make sure they are correct. Also, did you use the "scale recipe" feature to change to 2 gallon boil? That is the way to scale your recipe.