You may have already experienced the problem first-hand. Have you ever tried to make a color print on your own printer, and found, much to your frustration, that the print you got looked nothing like the beautiful image you saw on your screen? It's because your monitor needs to be calibrated to ICC (International Color Consortium) standards. Your printer is already a calibrated device, but your computer monitor is not. It's not sending accurate color information to your printer.
It's the same way that those HDMI TV's at the department store all look a little different from each other, even though the same video is being shown on them. They're not calibrated to a common standard. So imagine then if you open your images on your computer and prior to sending them off, you make a series of adjustments that make them look better on your monitor. Your lab, which very likely maintains well-calibrated systems, sees something entirely different from what you see, making accurate color determinations all but impossible. And there's no profit in re-makes.
It's all about the color!