i think it has to do with the colors/materials chosen being too Vibrant/shiny or something.
using the Thumper as an example, it reminds me of a car fresh off the assembly line with a thick coat of new paint. that paint makes the material on the surface look completely different than what it really is so you dont see it as metal you see it as whatever the paint was designed to make it look-be it matte, pearlescent, or metalic.
personally i like it, as it reminds me of sci-fi materials like Plasteel and such. although now that i think about what i just typed wouldn't it be awesome to offer different types of paint in game so you can change the appearance of the materials even more?