Some good points from Scott and Bart. How is the file submitted? Almost every file submitted for this type of processed is resized and the saved again (another jpeg compression). If you submit a file to exact resolution it will print at the correct size, but it will be slightly uprezzed so it can “bleed”. This is how they can make borderless prints (not unlike any other printing process out there).
Most of these printers can handle the file in it’s original resolution their software can resize it to the final size. So if you send the original size you end up only getting it resized one time in the process, not once by you and once by the printer.
So it depends on what process the lab uses and how the file is submitted. In our Chromira we have hot folders for each size, and if someone sends a file we drop it into the appropriate folder, and the printer handles the resize - we don’t load it up on a machine, resize it, and then submit that (something which some small labs do). We also accept orders through ROES, where the original file is sent as well as instructions as to what to do with that file.
While submitting exact size files rarely presents any problems we have seen some rare odd banding that is resolved by sending the original file at it’s original resolution.
Another problem is if the jpeg quality level is set low. for high quality work be save and jpeg to the maximum quality level.