The IBU's are determined by the amount of sugars, volume of water, the amount of hops, and when you added them.
If you didn't use all of the sugars (grains) it'll increase the IBU's as it's not there to balance them.
So if you had a 4 lb bag of extract and X number of hops, and according to the directions it would give you 45 IBU's, it would be something like 50 IBU's if you used only 3.75 lbs. If you added an additional 1/4 lb it might then be 40 IBU's. Same if you adjust the volume of water.