50% Gray in RGB, Lab ang Gray-gamma 2.2: why are different?

Hello,

not sure if this is the most proper forum for this question but my question has a lot to do with printed colors, feel free to redirect me to a most appropriate forum in any case.

I have bought a ColorMunki Photo and started playing with B&W tones using spot readings.

I'm genereating some gray patches and measuring it.

Doing so I have noticed something that I'm not really able to figure out by myself and:

- If I create a new white sRGB image in Photoshop (working space sRGB IEC61966-2.1), select a square and perform Edit - > Fill using 50% Gray in the Content Use drop down menu (blending mode normal and opacity 100%) I get a gray patch of R 128 G128 B128 values. And that's Ok for me.

- Now, if go to Image -> Mode -> Lab Color, and look at the info panel for the L value of the above patch I see L:54 (and not L:50)

- If I select another square and fill with 50% Gray (same procedure as above) I get a L:50 value for this new one.

- Then, if I go to Image -> Mode - Grayscale (using a Gray Gamma 2.2 profile as working space in the preferences) and and look at the info panel for the K value of the above two patches I see K:50% for the first and K:53% for the second

- if I make a third patch now filled with 50% Gray I get a K:50% value

- finally if I go to Image -> Mode -> RGB Color (working space still sRGB IEC61966-2.1) and look at the info panel for the RGB values of the above 3 patches I see 128,128,128 for the first, 119,119,119 for the second and 129,129,129 for the latest one!

Now, If I print this file I get 3 different Grays (CMYK values are different too obviously), but what is really puzzling me is that all these 3 gray patches were namely created using Fill and 50% Gray.

Who is the "real" official reference 50% Gray one of these 3?

Please, can someone explain this mismatch and/or point me towards some learning references online?

Many thanks in advance for the attention.

Ciao