I remember reading that about half of people in Miami were deficient in vitamin D. It makes some intuitive sense if you consider that our bodies evolved in conditions that made us exposed to sun quite a bit more than even people living in sunny places, in good part because we spend so much time inside buildings (A/C helps) and with clothes on.

There's also a difference between not being deficient in vitamin D, and having the optimal amount of vitamin D available. People tend to confuse the two, as if as soon as you aren't deficient you can't possibly improve on that.