When you order a photo in, say, 10×15 cm, you might expect this print to be exactly 10.0 × 15.0 cm. In reality, it is often just a little larger. For example, a 10×15 will usually be 10.1 × 15.2 cm. 

This is not a fault in printing, but has to do with photo paper, proportions and the way it is cut. Photo paper is produced worldwide in standard rolls, where the width and length are just slightly larger than the name of the size indicates.

Why is the format just different?

• Paper standard: photo formats originally came from the analog era. Back then, they were made on rolls of paper of fixed widths, making the dimensions slightly larger in practice.
• Cutting margin: printing and cutting slightly wider prevents a white edge from showing when finishing.
• Aspect ratio: digital photos usually have a ratio of 2:3 (such as 10×15), but depending on your camera, this may vary slightly. To preserve the whole photo, the size is sometimes adjusted slightly.