According to an article titled Understanding Bitterness by Stan Hieronymous in the Aug-Sep issue of Craft Beer & Brewing, adding spices can change both the actual IBUs and the perception of bitterness in beer.
Trials conducted several years ago by the Boston Beer company included adding 3.8 grams per liter of several spices for seven days to a base beer calculated to have 20 IBUs. Spices used were coriander, cocoa nibs, dried lemon peel, coffee beans, and cinnamon, and were added individually.
The control sample measured 26 IBUs. IBU levels for some of the rest were as follows: coriander, 27; cocoa nibs,24; coffee beans, 29; and cinnamon a whopping 80 IBUs.
More amazing was the perception of bitterness by twelve trained tasting panelists. Despite the massive increase in IBUs measured by lab equipment from the addition of cinnamon, three of the twelve didn't find it more bitter. And the panelists split on their perception regarding the cocoa nibs addition: six rated it more bitter and six less bitter.
The author did point out the an individual's genetics play a role in how tastes are perceived.
The above covers only a small portion of the topics and useful info in the article. I urge you to read it for yourself.