Magnesium is a cofactor for proper D metabolism.

Vit D is what is called a pro-hormone.

This explains in more detail:
http://en.wikipedia.org/wiki/Vitamin_D

A hormone is a chemical molecule made in the body that is released at a distance to affect metabolism of something or to enable something. We think of hormones as "drugs" because cortisol, and sex hormones are frequently given this way. But hormones are also very diverse and do many jobs all over the body. In this case Vit D is synthesized in the skin. It is converted in other organs to do specific jobs. Much like thyroid T4 hormone is synthesized in the thyroid and then moves around the body and is converted at tissue sites when needed to active T3.

Vit D was called a vitamin back when "deficiency" studies were conducted on animals to find out why certain diseases occurred with various dietary malnutrition. Pellegra and niacin, Beri Beri for thiamine, scurvy for Vit C... etc. Vit D was linked to rickets back then. But as time as passed, and the biochemistry figured out, then the actions of Vit D and its structure, was discovered to be non-vitamin in nature.

What happens with nutritional serum testing is that 1) it may not reflect much ---red cell testing is better, 2) doctors still often misinterpret the results because of antiquated lab ranges, and also sketchy training in preventive medicine and nutrition.
Lab ranges were made based on statistical "normal" people...but at the time, not much was understood about what was "normal". Also many nutrients were never given ranges during supplementation. So naturally taking things, would raise a basic "normal" range out of that range because the test subjects were taking NOTHING. Very high and very low ranges in electrolytes like magnesium reflect disease, but the large middle "normal" range can really be useless. What is needed or normal for one person is not necessarily normal for another. That is just an example for magnesium.