Glad your wife is now getting her Vitamin D deficiency corrected. And it is possible that she has not been at her current low consistently, due to differing levels of sun exposure, etc over her normal course of daily living.
It is important to understand, also, the diffence between acute changes biologically and chronically developing changes. Take, for instance a woman who as she gets further into perimenopause, begins to have progressively worsening abnormally heavy bleeding with her periods. She then also begins to have disturbance in the frequency of these abnormally heavy periods, as in having two periods per month in which now, instead of bleeding for 3-4 days, her period now lasts for 7-8 days. I've seen a number of instances of this in which early on, the woman goes to her MD and labs are drawn, exam done etc, and she leaves frustrated with the advice that her blood work is good, hematocrit and hemeglobin fine, etc, and if the problem gets worse, a d and c could be done, or hysterectomy etc, but that the problem is not at that level now and the woman isn't too excited about those options either. "wAit and watch."
So, this goes on and on and maybe gets worse... (I have known any number of women in whom this got to the point where they literally had difficulty being at work as they were bleeding so much they were frequently having "accidents" and having to make many bathroom trips.)
Now, cut to the chase, say 8 months post MD visit...woman goes back to MD as nothing is changing. This time bloodwork is done and holy smokes! She has an H and H that is that of a person with an acute, life threatening bleed out whose pulse would be through the roof and BP through the floor, unable to stand, function, etc and who needs a transfusion NOW. And yet our woman, though she doesn't feel good and has low energy and may be pale, etc, does not have a significantly elevated pulse nor a bottomed out BP, though it may be lower than her normal and she drives herself to her appt and walks fine and has been working, albeit feeling more and more exhausted.
The difference is that one process is very rapid and the other is gradual. In the rapid onset, the body has no chance to adjust - equilibrate. In the chronic process, the body adjusts slowly and the person does not experience the acute symptoms that they would have easily identified as a BIG problem, if the onset had been rapid.
This can be true for a variety of health issues, both in the realm of what we call "physical" health issues as well as what we call "mental" health issues. (I do not accept the split. In my book, all disease, all health issues =biopsychosocial.)
There are many other examples.
A sort of habituation can and often does occur with a slowly evolving process.