Beersmith is giving me the shits! (Warning IBU calcs inside)

[Nizmoose]

First of all sorry, I'm sure this topic has been covered many many times before and I've given it a search (I usually find the answer) but I'd love some experienced peoples explanations on this. Its a simple one. Why the hell does beersmith change its IBU number so much just based on changing the add after boil tick box? I know about hop utilisation I get all that but I don't understand or believe that the IBU's could change so drastically. As an example I have a simple extract recipe I was looking at using briess LME. By changing the add after boil option on and off I got an IBU swing of ~10 IBU's on a high 20's IBU beer. That seems ridiculous to me! So is it not stupid and the IBU will actually change that much or is there a recommendation in terms of setting to add before boil always for consistency or something like that? I revert to using IanH's sreadsheet because there you get what seems like a normal IBU that doesnt change. I mean surely 10g of centennial at 60 minutes wont swing 10 IBU's just because of boil gravity?

Heres some screenshots for reference:


IMPORTANT EDIT: I should also mention that the change makes no sense regarding generally accepted hop utilisation theory as the IBU jumps up for no extract added during the boil (so just water) which makes sweet F all sense

 

[warra48]

What volume boil are you using?

 

[Nizmoose]

The equipment profile it is set to there is for a 10L batch and a 13ish L boil

EDIT: Just to add I'm probably going to just do more like a 7L boil and top up, My plan was to aim for a 1040 boil gravity and go from there but with IBU's ranging from 28 to 38 now I don't know what to do lol

 

[Mardoo]

Are you using the most recent version, 2.2? You can choose steeping for hop additions added after boil and specify the length of time. Gets it a bit closer for knockout additions. It's definitely not perfect though, nor could it be given real world variation.