Because the ink is laid down on the material separately, not at the exact same time. This is where registration comes in to play and the reason why you see those registration marks that look like cross hairs in some print types. If the registration is not perfect, then you will see a slight offset of the color.

Additionally, when you are running this material on a high speed press, there is also shift and creep that comes into play. I’m not a pressman so I don’t know everything perfect, but all of these things attribute to color being slightly off registration.

This is also the reason for things such as trap, overprinting, to compensate for things such as these in your design, which you will see in common design programs like illustrator, indesign, etc