The simple view of the bonding in methane You will be familiar with drawing methane using dots and crosses diagrams, but it is worth looking at its structure a bit more closely. There is a serious mis-match between this structure and the modern electronic structure of carbon, 1s 2 2s 2 2p x 1 2p y 1.
The second reason for a low vitamin D is that you have primary hyperparathyroidism due to a small tumor in your neck and the body converts all the vitamin D-25 (that your doctor is measuring) to the active Vitamin D-1-25 (that doctors do not measure). This is the blue blob on the graph.
There are many possible causes of vitamin D deficiency. Limited exposure to the sun. You may look out your window and see the sun shining and think that you are safe from this deficiency, but that is not always the case. Even in sunny climates there is an increased prevalence of vitamin D deficiency.