I don't know how AH uses these mappings, because environment mapping means something completely different to me. In rendering the environment map is the image that is reflected on the surface of reflective objects, like the plane you are working on. However, it appears that you are working on something more akin to reflectivity mapping, which would be a percentage of reflectivity of the surface. The UV maps you posted indicate different reflectivity for the two models with the white insignia of the D model being nearly perfectly reflective (as it would work in my world anyway).
As to the amount of "dullness" it may be that you have one of the images in the wrong mode? Grayscale versus RGB for instance might cause an issue. Still, even the first B pony has a duller, more matte finish, by way of comparison. It should if we are taking into account a less intense reflectivity. I see no logical reason for that to happen as it is though.
It's interesting to me, because I have used many modelers (3DS Max, Maya, Lumberyard, Modo, Blender, etc.) and this is the first environment map that is done this way. Even the Lumberyard Environment map allows for an animated map in a way I presumed AH3 was using environment, but your post doesn't indicate that to be true.