The readout and the dial float over map tiles that move. Every other surface in this system contrasts against a colour we chose; this one has to contrast against a colour we will never see. The scrim carries all of the contrast — backdrop-filter carries none of it, because blur preserves local average luminance and a blurred white sky is still white.
What you are looking at: the same panel over eight known backdrops, including the two the arithmetic says are hardest — a blown white sky (worst for the TEXT, because it lightens the composite most) and mid-grey #5F5F5F (worst for the EDGE, because it is far from both black and sage at once).
The brightest fixture Darkroom ships, edge luminance 144/255. A photograph is not a flat colour: the scrim has to hold against every pixel under it at once.
Read from getComputedStyle and the known backdrop, per panel. The pixel-level check — screenshotting and reading the ACTUAL composited RGB, to confirm the browser composites in sRGB the way the arithmetic assumes — is npm run panel.