You are somewhat right: the rightmost image is not the sum of the three images to the left of it. It's easiest to see that the rightmost image doesn't even have the ambient contribution on the left-hand side of the central spheroid.
I've done an experiment with two modes of addition of the three images. In the resulting images below, on the LHS, with the label "Addition", is my result, on the RHS is the rightmost image from the OP.
- Simple, naive addition — as if the images were in linear RGB space (and retaining the result in this space). Here's the result:
- sRGB addition, i.e. the addition where we consider the images being in sRGB space (the default assumption on the web when no color profile is present), and so undo the sRGB gamma before actually adding color values, and apply it back afterwards. This will give the following result:
So, you can see that both ways of adding colors result in a sharp border from the ambient lighting contribution — the leftmost image in the OP, while the rightmost supposed result doesn't have such a border on some sides.
I'm not really sure how the rightmost "Phong Reflection" image was created. It may have been manipulated, e.g. some gamma adjustment or something like that. But in any case, in both ways of addition above the image didn't get overexposed as you supposed it would.