Beersmith - lowered LME amount and IBU's increased?

[AmericanCracka]

Using Beersmith:

Original= 6.6lbs Pale Malt, Maris Otter [Boil for 60 min]
IBU = 98.1

Change:
3 lbs 4.8 oz Pale Malt, Maris Otter [Boil for 60 min]
3 lbs 4.8 oz Pale Malt, Maris Otter [Boil for 0 min]
IBU = 160.4

That is the ONLY change.
Why would my IBU go up?
Thanks!

[grathan]

That's quite a change. Is it because the boil pot is so small? maybe you could export and post the recipe file here.

[Scott Ickes]

Did you enter it as Pale Malt, Maris Otter as you show in your original post?  Or did you enter it as LME?

The lower the gravity of the wort, the better your hops will be utilized.  In other words, you will get more IBU's from the same hops in a lower gravity wort.

Why it is soooooooooo much different, I don't know.