When we print your files we print at a slightly larger size so that when we cut the prints off of a sheet there will not be any white borders. By doing so your background/images will bleed out to the edge.
When your prints are trimmed down there is a margin of error we call the safety. By keeping all important information from getting cut off we require a 1/4" of safety. That's 1/8" on the top, bottom, left & right. If you have any questions on measurements, be sure to download the template for the specific product you are designing to ensure that you have the proper trim & safety.