Why changing the normal affects the viewDir?

First, I would like to let clear that this shader makes no sense. It's just the problem of my real shader isolated.

 Shader "Test" {
     SubShader
     {
         CGPROGRAM
         #pragma surface surf Lambert
 
         struct Input
         {
             float3 viewDir;
         };
 
         void surf (Input IN, inout SurfaceOutput o)
         {
             o.Albedo = IN.viewDir;
             //o.Normal = half3(0, 0, 0);
         }
 
         ENDCG
     }
     
     FallBack "Mobile/Diffuse"
 }

Apply this shader to an object and move the camera/view port around. You will see the object changing color. This happen because the RGB is mapped to the relative camera/view port position. That's fine.

Uncomment the commented line and the colors will change. Of course we would expect a lighting change, but what happens is a hue change, what makes no sense for me.

So, why changing the normal affects the viewDir?