Why did this crown moulding not fit?
First off it is not the best idea to assume that the walls and ceiling angles are all at 90 degrees. These should be measured so that you can make minor adjustments in the cuts if necessary. 

The primary reason that the cut did fit is because the first piece of crown molding was not installed correctly. It was pushed too far flat toward the side wall in the direction shown by the green arrow in the following picture.