I'm assuming it's the mitre cut and not the coping. Mitre cut was with a hand saw and mitre box at 45 degrees. I'm also assuming the ceiling is 90 degrees to the walls and the walls are 90 degrees to each other. Why is this cut so way off?
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.
In addition to comment above, which is entirely possible that walls are not square, with dimensional trim like this you need a compound mitre. It's not just cutting flat at a 45 degrees, you also need a tilt, called bevel angle, that corresponds to the dimensional depth of the trim. Crown is one of the more challenging to get the right "tilt" on. Here is a youtube video that might help you understand:
This is why we use miters for crown. In this case, you needed a test piece that was coped on the right side. You roll the crown moulding up and down with the test piece over it. When there is no gap, nail the right side. This assures the next piece will fit properly. In this case, the right side needed to be rolled upwards toward the ceiling and then nailed.