Noise reduction amount - global vs. mask settings interaction?

I've been trying to understand better how the global luminance noise reduction slider values (0-100) interact with a mask luminance noise reduction slider (-100 to +100). In all cases "NR" here is just basic luminance noise reduction amount, and I'm leaving the global detail/constrast at default 50/0.

My understanding/expectation was that the global and mask NR settings were directly tied to each other. So, they would essentially net out on a 1:1 numerical basis.

If that were true, I'd expect that global NR 50 and a mask with NR -50 would result in zero net NR in the masked area. I.e., no difference in the image in the mask. That's not the case. Adding the NR -50 mask "sharpens" the masked area back up to some extent (removes some of the blurring effects of the NR), but it is still less detailed/blurrier than the original. There is some effect from the NR at play. Even dialing the mask NR adjustment down to -100 doesn't get me back to the original overall pre-NR state.

Does anyone know the actual process going on here? Is the global NR is applied and calculated, and then a separate... "negative NR" process is applied afterwards on the resulting image by the mask? I.e., it's doing two separate calculations?

If so, what exactly is that mask-based slider doing—what is "negative noise reduction" as compared to, say, sharpening? Just a literal inverse of whatever the NR algorithm is? How do the -100/+100 mask scales correspond with the 0-100 global scales, if at all? Even if it were 1:1 as I had originally expected, the different scales numerically leave the possibility for this "negative noise reduction."

Thanks for any insight!

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Note: I've also tried this with sharpening set to 0 vs. set to Lightroom default 50 for RAW; no difference.

Using LrC v13.3.1 on MacOS Sonoma 14.5, but have had this question for a number of versions of Lightroom. Also doesn't apply solely to LrC, I guess.