There used to be a web page called "Your Eyes Suck at Blue". You might find it on the Wayback machine.
You can tell the luminance of each individual channel more precisely than you can perceive differences in mixed color. This is due to the difference between rod and cone cells. Your perception of the color gamut is, sorry, imprecise. I'm sure that you really can't discriminate 256 bits of blue in the presence of other, varying, colors.