Can someone help me out on this one?

A Lab luminance value of 50% in Adobe RBG space equals RGB 118,118,118 and a brightness of 46% in HSB.

Now aRGB is a perceptual uniform color space with gamma 2.2, so why isn't a luminance of 50% situated at RGB 128,128,128? Does this mean that aRGB is not as uniform as Lab?

When I do the math on 18% reflectance I also arrive at 46%: 0.18^0.4545=46% (0.4545 = function of lightness sensation, the inverse of a gamma of 2.2). Or am I mixing up things?

Thanks

Herman