When you go through nether portals it puts you that same x,z coordinate, rather than going the coresponding nether portal. basically you don't appear in the linked nether portal. If you were to enter the portal in the overworld at x=800 y=64 z=800, rather than entering the nether at x=100 y=64 z=100 you would enter at x=800 y=64 z=800. the same thing happens but in reverse when leaving the nether.