There is some ambiguity about this so I will try to shed some light on it.
Aight sooooo IBUs are International Bittering Units which basically measure the amount of isomerized alpha acids in solution. IBUs are not the measurement of actual bitterness of a beer. Isomerization occurs when energy is applied to alpha acids (in this case energy = time, temperature, & agitation) - the more energy = the higher the utilization of the aa = the more potential bitterness is added to the beer in IBU. For example, the longer you boil the hops the more bitterness you extract (you know this).
Many brewing softwares attribute 0 IBU to a hop addition if it is post boil (so whirlpool additions + rest/hop steep), but this is not the case because the wort is still very very hot (over ~170 F for isomerization to occur), and it takes a while for you to cool it down. At my brewery, we get nearly as much utilization out of our whirlpool hops as we do a 30 minute addition - this has been measured).
Most breweries just rely on different IBU calculations and models to determine their theoretical IBU, and don't actually get the beers measured. All the current models fail pretty miserably when trying to calculate IBU contribution from post boil additions.
There are many more compounds in hops that are bitter or contribute to beer bitterness, besides the isoalpha acids/IBU. Also, the way you associate hops, and the way the beer finishes, also contribute to higher or lower perceived bitterness. If the beer finishes drier, you will think it's more bitter. If there is more hop flavor/aroma, you generally perceive it as being more bitter (and if it were dry hopped, it will be more bitter... just not necessarily higher IBU).
My guess is the brewer did not do any boil additions. They MAYBE threw little/moderate hops in during the whirlpool, but they DEFINITELY dry hopped the hell out of the beer in the fermenter.
