Perceived IBUs and measured IBUs are two totally different things. Perceiving is pretty simple, we just taste and decide how bitter it seems.
Measuring IBUs (with a spec) involves extracting compounds with solvents and measuring the transmittance of light through the liquid. You measure a blank solution with no bitterness, you measure the sample from the beer, you determine the difference and apply some factor to determine how many "IBUs" are in the beer. The problem with the method is that there may be other things in the solution that affect the transmittance but that are not bitter, or not as bitter, or may be even more bitter than the standard "IBU".
In the case of FWH, it is possible (and maybe likely) that the extended time or different temps may create compounds that are different enough from the iso-humulone compounds to be perceived differently but not be measured differently.