It is hard to reconcile the fact that it's so easy to get enough D from sun in a short time with the fact that deficiency levels are roughly 40% in the US, with

It is hard to reconcile the fact that it's so easy to get enough D from sun in a short time with the fact that deficiency levels are roughly 40% in the US, with insufficiency (20-30ng/ml) much, much higher than 40% (and also high is much of the rest of the world). Look at the world chart in the paper (copied from Palacios et al review): 73% insufficiency in Austrelia, 61% in Britain even in summer, etc.).