I used the same nodes there. Just different settings. Here is a diagram that I hope explains the issue.

Suppose the black line represents real-world behavior of skin, with respect to apparent brightness based on the lights. A perfectly realistic shader that worked under all lighting conditions would track that line.

But to do so would take a lot of math nodes. Suppose instead you don't feel like doing that and just want to use a single math node (Add) that produces a straight line. The Add node has multipliers built into it (the two numbers), so you can produce any line you want. For example, to implement the line y = mx + b, you make a math function Add. Plug "x", the value you're getting from somewhere else, into Value_1. Put "m" into the number for Value_1. That makes m * x. Now put "b" into Value_2. That computes mx + b. Clearly you can make many different lines by chosing m and b.

In the diagram, I show two choices for m and b - one is colored red, the other green. Neither one looks anything like the black line except for a narrow range of x. Depending on the range of values for x in your scene (light brightness) you'd want to adjust the approximation so that the area where it best overlaps the black line is in the neighborhood of the x value for your scene.

Does this help?

The last picture you posted was using actual shaders which I uploaded - you can download those exact shaders and look at them to see what values I chose for the parameters.

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