HDR: Why so many exposures?

If the sensor on my camera has 8 stops of dynamic range, and the scene I want to photograph has 16, then, in this extreme case, I only need to take two exposures to capture the entire dynamic range of the scene. However, capturing more than two exposures tends to yield better results. Why is this?

I appreciate all responses, but am really interested in a more quantitative/technical/mathematical answer.

Thanks,

Tim