Here's a quote that indicates what I am feebly trying to articulate. "One obvious result from this experiment is that IBU values can not be directly substituted for isomerized α-acid values, especially at short steep times, high hopping rates, and sub-boiling temperatures. This is because IBU values reflect not only isomerized α-acid values, but also contributions from oxidized α and βacids and polyphenols."
Um, soooo lab measured IBUs contain more than just isolmerized alpha acids. Like I said, whirlpool hops do bring bitterness to the beer. More than an isolmerization chart will predict.
Saying that IBUs only come from isolmerized alpha acids is like saying high alpha hops are for 60 min additions and low alpha are for dry hopping. Ya, maybe a good starting point but keep experiencing and adapting to your experience