There shouldn't really BE a difference in how they check your result. The difference would be in HOW they decide to treat it (that's if treatment is needed).
Since your GYN has apparently decided you no longer need to be on the prescription vit.d, then it might be a good idea to re-check with the original internal medicine doc who was treating you before.
According to the lab which checks my blood (Kaiser Permanente), the range for vit. d-25 hydroxy is 30-100 ng. (some labs may vary on this). So even when you were at ''32'' you were still within the range (albeit the low end of the range). As to why you dropped from 52 to 37.2 that would also be something to discuss with your original prescribing doc. Your GYN SHOULD have taken your previous result into consideration!
You may not have to continue taking the supplement, but I'd get a few different opinions if I were you.