Here is my take. The seam is leaded at the factory by some craftsman who did this job 8 hours a day. My guess is that the car received whatever the shape the worker put in when he did the job. He was probably supposed to have a squared off corner per some specification. But I bet it was much harder to shape the squared off corner than it was to round it out. Especially as you are doing the work on a line where the cars keeping coming and going.