"All" four columns should display distinct graduations within thier column when properly calibrated. Windows Gamma is 2.2, MAC is 1.8.

Note:

Gamma is a simple type of Tone Response Curve. Tone response is a description of how the monitor’s output luminance relates to the input signal. In practical terms, tone response mostly affects the lightness of the middle and quarter tones of images displayed.

Generally, higher gamma values (e.g. 2.2) cause images to appear with more contrast and color saturation – usually with a loss of shadow detail.

Lower gamma values (e.g. 1.8) cause images to appear with less contrast and saturation but with more detail in the shadows.

Color managed applications adjust for monitor gamma, showing similar results at a range of gamma choices. Gamma 2.2 also provides a perceptually uniform tone range to the observer.

Source: Datacolor, Spyder4Pro Documentation.

For Photoshop CS4 processing I'm processing in Wndows RGB at Gamma 2.2 but switching to Macintosh RGB Gamma 1.8 via "View > Proof Setup" for better visualization of faintest portions of my images.