I have seen many Benz's with AMG on the truck and it wasn't a 63(C63, E63, S63) class car.

Chances are those are people slapping the AMG badge on after purchase. There are a lot of Benz owners that buy for status and if they can't afford a real AMG they add the badge themselves to look like ballers. I would bet all the MB's you see that aren't 63's, 55's are fakes. Unless someone can tell me positively otherwise, I don't see MB putting the AMG badge on a regular series car. That would cheapen the AMG brand.