Those figures, based on my experience look a bit "convenient", and perhaps like some marketing……as in exactly one meter radius difference per ski length difference in the skis? Hmmm. Not sure if that math works, but that's above my pay grade.
My hunch is that the places where I saw 15M probably took the "close enough" approach. BTW, I have two pairs of GS cheaters, and have had a number of others. All in the 178cm to 180cm range. The same ski, made by the same company skis a lot differently in a 185cm versus a 180cm, for me. It's got more to do with the layup, I believe, than the radius. I've skied a bunch of them, and have clear preferences, but have never gotten too caught up in the exact radius.
Regardless of whether it's 14.8M. 15.2M, or 15.5M in the 178cm, I doubt if Marchel Hirscher would notice the difference. I presume that you're not making the length selection based on the radius, but on other factors as well….your size, etc.
Good luck. Hope I didn't confuse things…..