There's a couple reasons. One is that AA% varies from one crop to the next, so any estimated IBU for a kit recipe is just that: an estimate. The other reason is that there are a few different formulas for calculating IBU: Rager, Tinseth, and at least one more I can't remember off hand. If you go into tools and change the formula from one to another you will get a slightly (in some cases not so slightly) different answer. But really, anything over 100 IBU is not detectable or accurate, so you're good regardless. FWIW, according to one of the homebrew sheets Vinny has circulated online, Pliny has been chemically tested and found to be somewhere around 90-95 IBU, which is really the only truly accurate way of measuring it.