I agree with you on this. I have a couple early 15" orders and one on them is borderline on too tight while the other is perfect. When laid down flat and measured, they are both spot on at 15". My shirts since then are 15.25 and that seems to be just enough to solve the problem.
I've always assumed the variances in cloth thickness cause it. But if you try to prove it with math (circles and circumferences, etc.) it takes a pretty big collar thickness to add up to an additional 0.25" circumference. This makes me assume there are other variables to consider. Perhaps for example how well the neck band rolls around your neck without creasing, how tight the neck button is sewed, etc. It's easy enough to adjust a button to make a difference.