The thing is that we don't want contrast reduced too much. The ideal raw histogram has only the gamma curve applied (what Photoshop incorrect calls "linear"), no extra tone curve. If you reduce contrast (inverse "S" tone curve), you make it seem like values are lower than they actually are, and it's even possible to have clipping hidden by it. The reverse is obviously bad (normal contrast-enhancing "S" curve), because it shows clipped highlights where there are none, and makes values higher than they actually are.


On Nikon, -2 or -3 contrast is the point where contrast is actually neutralized, and you're getting just (or mostly) the gamma curve. But on Canon a setting of 0 is neutral contrast. I could be wrong, though.