If you read Palmer's Learn to Brew, check out Chapter 5 on Hops. He discusses utilization. Basically, what he boils it down to is that the volume of wort and the gravity of the wort, along with the amount of time you boil the hops go into determining the utilization.
He uses a scale developed by Tinseth to give utilization of hops boiled in a wort of a certain gravity for a certain amount of time. The math he uses is fairly straightforward. You determine the AAU contribution of your hops by multiplying the weight of hops you use by the AA% for the particular variety.
Then you determine the gravity of the wort in your boil (he gives instructions for that as well).
Then you look up the utilization on the Tinseth chart by taking the gravity of your starting boil and comparing it on the chart to the amount of time you intend to boil the hops in that gravity.
Finally, you multiply the AAUs contributed, the utilization and a constant for metric/standard conversion (explanation in the chapter). Then divide it by your final volume (batch size) to get the number of IBUs contributed by each hop.
So the short answer is ajdelange's. You get more utilization (and therefore more IBUs) boiling your hops in a larger, more dilute volume of liquid because, as Palmer states it, "hop utilization decreases with increasing wort gravity".