In some conservative Muslim countries -- often found in sunny areas -- girls have high rates of vitamin D deficiency because of the practice of being very covered up anytime they leave the house. I read some book authored by a woman from such a country and one of the details that stood out was a remark she made about how every woman who gets out of those countries remarks on how wonderful it is be able to just go outside without a veil and feel the sun on your skin.

Plus, as others have noted, some of those countries have very young populations and darker skin tones of local inhabitants impacts vitamin D production as well.

There are really going to be a lot of factors involved.