Extract add after boil IBU

I am Extract brewing and have noticed that extract added with the "add after boil" checked does not seem to affect the IBU of the recipe. I can add 1.5kg of extract or 15kg and it has no a effect. Is the IBU displayed just for the boil or for the whole brew?

The gravity affects the isomerization rate of the hops.  The higher the gravity, the lower the rate.  So, when you add extract after the boil the hops have already isomerized and thus no affect in the IBU. 

Thanks jomebrew,

I have mixed up IBU as a concept with perceived bitterness in the final brew. Rookie mistake!
The software really helps to record and feedback on what you are doing whilst learning, especially with the help of fellow brewers, thanks again

mark