That is because the difficulty is loaded from the code, so you dont play all chapters on normal to get good scores, then play the last one on insane and get a good overall score. There might have been some other reason TheSpoon did it that way that I'm not remembering, though.
At the moment there is no way to do that just by playing the map. At some point that may be added but would make tracking the scores a bit of a hassle, and it isnt something too important.
I guess, if you post the code I could change it to insane