The BJCP IBU ranges are based on lab results and brewer reports. They're not calculated. Similarly, OG is calculated from the stated alcohol level reverse engineered into the bottle gravity.
iBU Calculations are estimates to get you into the ballpark. The various calculations have tried to adapt general homebrewing practice into a formula. The most accurate formula is the BU:GU ratio which is a statement of relative balance that is the mean of the style. The closer your numbers are to that ideal, the closer your balance is to the center of the style.
The reality is that hop isomers are doing all they can to escape your beer. They're hydrophobic. So, they love foam, fermenter sidewalls, tubing and any other surface they can adhere to. An overly vigorous boil or one that goes too long can break isomers down as fast as they are created. Yeast selection plays a role because some strains absorb more than others.
And the list goes on. People have earned Ph.D.'s and lived entire careers studying bitterness in beer.
Bottom line, you'll get a sense of how hoppy your beers turn out with any single formula when you taste several batches. If one formula lets you get closer to the ideal BU:GU ratio across most of your beers, use that one and forsake all others. In other words, if you need more bitterness, just add more hops.