IBUs for First Wort

[Craig Wilkins]

Question: How come First Wort Hops adds less IBUs than the equivalent added to the 60 minute boil?

I read a recommendation to try splitting your 60 minute hops addition (first addition) into two.

Put 50% as a First Wort addition and 50% at the 60 minute boil, in lieu of using all at the 60 minute mark. When I did this, the overall IBUs went down, which confused me in my Brewer's Friend recipe.

Can someone help me understand how this works?

 

[Ozarks Mountain Brewery]

its a smother bitterness than being boiled due to adding the oil before the boil, it actually adds more flavor and less sharp bitterness, I even do mash hoping regularly where I recirculate with my pump and add the hops to the mash and that even goes farther towards flavor and less bitterness, its a perception rating not an actual numbers for bitterness, you perceive this as being less bitter

 

[LlewellynBrewHaus]

My understanding with the theory behind FWH is that some molecules from the hops bind with maltose molecules and render the new molecules formed un-isomerizable
Therefore unable to contribute to the total IBU's . These new (flavor) molecules are later liberated by the yeast cells or on the palate when consumed

That's what I think I know about the subject hope it helps
Prost!