unforseen expenses = poor planning
I have to disagree. I cite the fact that EVERY large construction project that I have ever heard of has gone over budget and the more politically involved it is, the more leeway it seems to have to bust the budget entirely!
In light of the above, my view is that the planners start with the absolutely most optimistic scenarios possible. While they have to budget weather delays, they choose the least possible. While the project is new, they push building portions of the project that make it 'impossible' to remain unfinished. Once that is accomplished, the cost over-runs start to come in.