both the CastlePoser tutorial and a simple V Test gave me that idea:

http://www.castleposer.co.uk/articles/maths_bias.html

as far as i can tell from the tutorial's picture, the result of Bias(U, 0.8) has a transformed black isn't touching the bottom any more. even if i thought i couldn't trust the material room preview, which has a darkest color of 46, 46, 46, i have to believe this image.  unless i'm missing something?

going below 0.5, white doesn't seem to be transformed.  i totally don't get it.