boil volume effect on ibu

On brew smith checking out my recipe and noticed something I was wondering if anyone could help me with. My batch size is 5 gallon and I do a 2 gallon BIAB (steeping grains) then when done steeping I bring those 2 gallons to a boil and that 2 gallons is my boil volume . Whats weird is that on brew smith It tells.me my boil volume should be 3.67 but I use the extra water to cool my wert down quick by adding the other three gallons to it after the boil. When I adjustthe boil volume from the 3.67 to 2 gallons on beer smith my ibu drop from 24.7 to 12.7 and I was wondering why the ibu would change so much by having less of a boil volume even though the tottal batch size is still 5 gallons?

Is it adjusting your hops ratio at the same time?

With lower volume you will have a higher concentration of sugar therefore decreasing the isomerization of hops. Larger volum less concentrated better isomerization.

No the hop ratio stays the same

At steelers77 that make sense thanks for the help

At steelers77 . You are talking about just the boil volume and not totall batch volume

Yes the boil volume. It is advised for brewers using a partial boil to increase the hop rates to account for higher gravity.