Hoppy Irish Stout?

[Gusmedic]

Last week I posted this in the extract brewing section and didn't get assistance. Thought I'd try it here. Any help would be appreciated.
I brewed up an extract kit from Northern Brewer for a dry Irish stout. Going in auto-pilot mode for most of my lighter color, hoppier brews I only added about 2lbs of the golden extract and all 2oz of cluster hops at the beginning of the boil. I added the last 4 lbs of liquids extract at flameout. I just entered everything into beer smith and it's telling that by adding the hops into only the 2lbs of fermentable, my finished IBUs will be 69. Almost 20 higher than if I would've added all the extract at the beginning. Is this accurate? Did I mess this style up pretty good?

 

[BobBailey]

Hop efficiency goes down as gravity goes up, so doing a late extract addition does increase bitterness. Also, did the original recipe call for all of the hops at the beginning of the boil or some later on? Hop efficiency for either application is not linear, but can make a big difference in bitterness.