What am I not understanding correctly about luminosity and curves?

Either I'm missing something (more likely) or these features are bugged (probably not).

Firstly, I'll be using some example screenshots here that serve the purpose of isolating the behavior I'm referring to to an extreme degree that obviously wouldn't ever be used in normal use. Just putting this here to avoid "but why would you do that" kinds of responses.

I've set the tone curve as such:

To my understanding, this should (approximately) render a point near 50% luminosity as pure white and everything else as pure black (with a steep curve on both sides). And it appears to have done so. I only do this to make it obvious what the tone curve seems to think 50% is.

However, when I then go and make a luminance mask, targeting a range around 50% highlights a completely different area:

In fact, to highlight that same area, I need to instead focus on a range that centers somewhere around 70%:

So what am I missing here?