I'm not sure where you're measuring color saturation from, but it seems irrelevant in any case. Color saturation is not a measure are how light or dark a color is, but rather how saturated with color vs how "gray" it is. For example, a color with a color saturation of 0 could range anywhere from pure white to pure black, including all shades of gray in between, but no one would suggest that every shade of grey is equally intense.
Look at the 1.48 square for John Roberts 3rd from the right. It has an RGB value of (213,149,149). Conversely, the -1.48 square for David Souter on the far right has an RGB value of (228,233,244). Clearly the latter is far, far closer to white than the former, which should come as no surprise considering that it's quite obvious to the naked eye as well. Hell, look at the 2nd square for Earl Warren (top row). It's -0.45, which should place it slightly on the liberal side, yet it's colored a light pink. It almost appears as if the color scale is centered around something like -0.80 rather than 0. Either there are some fancy calculations going on that are not well explained and thus quite deceptive, or the colors are blantantly biased to make the reds darker and the blues lighter.