Maybe that's simply a limitation of that particular laptop or that particular laptop gpu? Because the laptop designed around an 8 bit screen it may simply be designed to only push that specification even if you are using an external monitor with it, it's something you'd have to look into thru either the laptop maker or someone that has a good understanding of this particular field.
I'm pretty sure there's a few knocking around on these forums but I forget who.
The laptop screen is 6-bit.
But I'm so tired of this particular laptop, that finally I will get rid of it. It simply doesn't work for me - more disappointments with it than good things.