It's the Gamma Correction kicking in that's doing that (which can't be disabled AFAIK). If you test the values in a photo-editing app you'll find HEX to be a couple of shades lighter than the RGB and HSV numbers. An RGB of R0.5, G0.5, B0.5 equals an HSV of H0.0, S0.0, V0.5, which should equate to a HEX value of #7F7F7F or thereabouts. But, it actually gives you #BCBCBC which is a couple of shades lighter - it works about to be about 27% black (light grey) instead of the 50% (mid grey) you'd expect with those RGB/HSV values.