Large service centers just have very streamlined processes. Lastly, the process is not heavily scrutinized. In other words; there's a lot of quality control at a factory, where something is new and should be close to perfect. On a used watch, an old watch, the quality control is less extensive. And if it does have a scratched plate, a chewed up screw, it begs the question if it was done by a previous servicer... You may be lucky. But you may also have scratches that you're simply not aware of. For instance, I wouldn't be surprised if some of my watches have big scratches on plates that I can't see. Things you can see from the sapphire case back are always more carefully treated because they know the consumer may potentially see it. But, the things you can't see, well that's all fair game.