There is a limit to the solubility of isomerized alpha acids, and the limit is affected by wort gravity. The yeast will also pull some out when it sediments. The formulas are fine up to a point, and above that they're just wrong. I wouldn't trust any calculation over 80 IBUs (and the higher it is the more it will be off), but have not tested it directly. The taste is more important than a calculated number anyway.