Because they're (idealy) not linear, I'd assume. Gamma correction[0] is an exponential-like function. Hence, sRGB vs "sRGB linear". Despite what many people think, sRGB color #808080 is not 50% the brightness of #ffffff, but about 75%. All because our eyes are non-linear.

Color math is a massive rabbit hole that deserves its own "things programmers believe about..."

[0]: https://en.wikipedia.org/wiki/Gamma_correction