I use a mini colour checker images for WB setting of raw images. The instructions say that the 2nd patch is best used for WB. That is 200,200,200 under the appropriate lighting conditions.
My Canon cameras both have custom profiles.
However, like you, I do get small variations on the RGB values after using the refine WB dropper and they change across the patch. I can only assume there is some variation across the patch or some variation in the light. I never get more than a difference of 5 across the RGB values which I consider insignificant. If I get more than that that I'll re-sample. In the end I may manually tweak to get an appearance that looks right.
Terry