Knowing modern medicine, it's quite possible for either case to be true. The test could be biased and the guidelines are misleading or useless for different demographics (I think any demographc that isn't upper class white male have at least one story to tell about this). Or that a population of people could quite generally benefit from more vitamin D.
I think it would be somewhat unsurprising if we find that genes that evolved for one climate are not ideal when migrated to another climate. Especially when we live much longer lives and these kinds of issues are much more relevant to our expected quality of life (ie. how would you know you're vitamin D deficient when you're a peasant who is everything deficient?) Which is all me just saying, I wouldn't be surprised if vitamin D deficiency, which is directly related to sunlight exposure, is a prevalant issue now that humans migrate so much more freely than ever before.
I think it would be somewhat unsurprising if we find that genes that evolved for one climate are not ideal when migrated to another climate. Especially when we live much longer lives and these kinds of issues are much more relevant to our expected quality of life (ie. how would you know you're vitamin D deficient when you're a peasant who is everything deficient?) Which is all me just saying, I wouldn't be surprised if vitamin D deficiency, which is directly related to sunlight exposure, is a prevalant issue now that humans migrate so much more freely than ever before.