See the effects of image saturation...

A direct comparison of NDVI and C3 Vegetation Index values shows that the C3 product is much better at showing differences in mature vegetation. This is because NDVI values saturate when vigor is moderately high to high, thereby making variability barely detectable.

Image Saturation

About halfway across transect A-B the vegetation vigor transitions from moderate to high. In the higher vigor area (circled red in above graphic) the vegetative variability is still apparent in the C3 Vegetation Index, but is almost completely washed out in the NDVI as the values are saturated.

 

transectc-d

The C3 image clearly shows the alternating high-low vigor in each row along transect C-D (you can even count all seven individual row peaks). In the NDVI image the saturated values again obscure the vegetation detail.