..... there is a case to be made that the outer dimensions be as per ICFRA - 2400x1800mm and 1800x1800mm. There are a number of other reasons to justify consideration of this. One is that it would reduce the target weight a little bit.
One Target Rifle reason would be for those that Frame the target when sighting. I used to do this past 600 yards. All of a sudden your ring sizes are no longer correct and would need to be bigger