That is because:
Firstly, It is not in game truck model, It is created by 3rd party.
Secondly: Its to do with the author and his skinning matter. Nobody can help you with. Either he set this to be like this OR he doesn't know about it.
However, I also think its to do with RBG to SCS sheme color coverter. And the lighting of the day.
OR MAYBE that is how originally the skin should have to look?
I hope somebody explain my point clearly because I can't explain in more detailed way
Thanks. Hope it helped