Confused With Info Panel

I created a new Document in Gray Gamma 2.2
I filled the layer with RGB 128,128,128
Placing a target point on the gray layer, the Info Panel Reports
RGB 128,128,128
K: 43%
HSB: 0,0,50%
Plotting the point with a Curve, the Input value reads 145
Can anyone explain why the K: value is not 50% and why the Curve Input in not 128
Ian
