A true photo is only when you shoot in slides. If you shoot in negative, even you tell the printers not to do any adjustment onto your shots, they just cant help because the machine will be doing the compensation and adjustment automatically. If the printer doesnt manual overide this actions, it is even worse cos the machine doesnt know what your photo is. Diff photo with diff tone and coloration.
Try take a digital photo and do all the auto level, auto contrast, auto color and see what happens to your pic.
Also, you PS your photo in your PC is not cheating. Some who develop their photos in the darkroom do some miracle work in the using their enlarger lens as well.