Another weird Beersmith quirk

[GreenwoodRover]

I just entered a recipe in beersmith for a kolsch.
Brew house Eff 70%
Finished Batch Size into fermenter 5.5gal
Lost to trub and chiller .5gallons
7# Pilsner
2# white Wheat
1# honey (add after boil box checked)
1oz Perle 8% 50min
.5oz Hallertauer 3.9% 15min
.5oz Hallertauer 3.9% 0min

I originally set the boil time for 60min abd the boil volume calc'ed to 7.62 gal and the IBU's at 30.6.
When I up the boil time to 90 min the boil volume goes to 8.36 (as expected) and the IBU's jump by a point and a half (not expected).
All the boil times on the hopping schedule stayed the same. I know it's not a giant jump up and definately withing the margin of error so to speak, but I just thought it was weird....

 

[anth]

Hop Utilization will increase slightly as your boil size increases. Mainly because the gravity of the wort decreases. More below.

How to Brew - By John Palmer - Hop Bittering Calculations

 

[GreenwoodRover]

Yeah I get that, but the hops aren't added unitl min 50 and 15 regardless of the boil size.
In theory, wouldn't the volume of wort be the same at 50 min regardless if I start with 8.36gal and boil for 40min before adding hops or I start with 7.62 and boil for 10min before adding hops?

Again I know it isn't a big difference, but after The Pol's experience here ( https://www.homebrewtalk.com/f84/beersmith-ibus-103683/ ) and the fact that my last IPA sent to a competition where Beersmith calced 65 IBU came back with comments declaring "not enough hops for an IPA, borderline APA as best" type comments has me starting to question my rote acceptance of Beersmith as very accurate...

 

[enderwig]

and the fact that my last IPA sent to a competition where Beersmith calced 65 IBU came back with comments declaring "not enough hops for an IPA, borderline APA as best" type comments has me starting to question my rote acceptance of Beersmith as very accurate...

Not enough "hops" doesn't mean not enough bitterness, adding 4 oz of hops at flameout adds no IBUs, but a ton of "hops"

I think your minor IBU adjustment for the larger boil is based off of the info found in Palmers book.