You ask a good question.
I’ll ask Cannell and see if he answers.
Perhaps this review article by Cannell and colleague might answer your question. The authors state that latitude is only one of many factors affecting vitamin D creation, and that vitamin D deficiency is common in Miami.
http://www.thorne.com/altmedrev/.fulltext/13/1/6.pdf
Dr Cannell’s email address is listed on the first page of the article.