It appears that your understanding is correct, based on the image I am examining.
I simply subtracted the prescanned norm data from the original data and the result was a smoothly varying field, brighter in areas closer to the coil. The FWHM (estimated via AFNI’s 3dFWHMx function) is ~63 mm, about a factor of 10x larger compared to the original data, for example. very very smooth.
The difference image was not identical across time, but varied by one here and there. Not sure what would have caused that, perhaps rounding at the level of image reconstruction. My testing data has a very low range, compared to typical 32 Channel data, so that may be to blame.
Thank you for the through answer, if I dig up some other, 'better 'data where I have collected both prescan norm and unaltered data I will update this post.