Right! Those two graphs are very similar. The difference between them is the slight bias towards a "lighter" print for what I would call the black and white preferred print study. How much "lighter"? The difference you pointed out by drawing a 45-degree line in red.
Here I am guessing the newer study of color reveals a preference for higher correlation to reality (in the mid-to-near-highlights), than in black and white which several authors have said should be "lighter". It makes sense to me that people would need "color" to be more accurate.
I also know people prefer my lighter prints, though I occasionally choose darker for my own artistic reasons (as an artist, each of us is entitled to break the rules any way we want).