Weren't they painted so they could be viewed from a distance, as many of them were not exactly eye-level. It's like stage makeup, you wouldn't want to apply the same makeup for performing in a play as you do... as normal.
I think there are a lot of different possibilities. As hinted to in the article, another is that the most evidence is left by pigments close to the raw surface isn't very well representative of the actual statue. If you're familiar with a lot of art processes - a base rough layer of paint is what is used to seal the raw surface and provide stable surface and rough background color sections for much more detailed painted features in following layers.
I think there are a lot of different possibilities. As hinted to in the article, another is that the most evidence is left by pigments close to the raw surface isn't very well representative of the actual statue. If you're familiar with a lot of art processes - a base rough layer of paint is what is used to seal the raw surface and provide stable surface and rough background color sections for much more detailed painted features in following layers.