Photo lab printers that use traditional light sensitive, develop-in-chemistry paper project your digital file via loads of LEDs onto the paper
below. To allow some margin of error for the positioning of the paper, your image is projected at a size that is a little bit bigger than the piece of paper it will be printed on.
Therefore some of the image on all 4 sides will be lost. In general, you’re talking about 1-3% of your picture. This is not a lot but when you have important detail at the edge of your image it can certainly have a negative impact.