Estimated IBUs

I just noticed last night that there is a check box on the liquid malt extract page that you can check to specify that the LME was added after the boil. Since I normally do some DME and hops boil, then add LME after the boil, I thought, 'Perfect!', but when I check that, my IBUs jumped from 20 some to 50 some. What? I was doing a simple little 1 gal hop boil of 2 oz of 6.9%AA hallertau for 20 minutes. Batch size after adding more water is about 2.25 gals.

Why does the estimated IBU go wonkers if I check that box?

Thanks
Monty

I'm not sure of the technical scientific explanation, but the basics are:

Hop utilization is inhibited by high gravity boils.  Basically, the higher the sugar concentration of your boil, the less bittering you'll get from your hops.  My adding the extract after the boil, you're telling BeerSmith that the boil gravity is lower and it is giving you a much truer picture of the actual bittering that is occurring.

Thanks. Yes I did a little research and found the Palmer section that states that. To me it seemed counter-intuitive, since you need to have some wort to get bitterness at all. I guess I got all caught up in the more is better thing. From reading Palmer, it seems like you need 1.030-40-ish for optimal bitterness. So I should go back and check that box, leaving it set to after boil, and adjust my volume to 1 gallon (from 1.25 as I just noticed) and maybe only use 1 oz or so instead of 2. That's all assuming about 1.5 lbs of DME in 1 gal for about 1.060 gravity. Sigh.. Gosh darn this beer stuff is like chemistry class.

Monty