Minecraft wiki entry for Nether_portals, associated text: Because of the coordinate change, portals created in the Nether are much more likely to have ideal coordinates that are horizontally distant from the Overworld portal that created them -- indeed, when used, they may create a new portal in the Overworld, rather than linking back to the original portal! When going the other way, horizontal coordinates tend to be closer to ideal (because whatever space is found will have a Nether-equivalent location closer to the original portal), but vertical displacement can still be an issue. Overworld portals that are within 1024 blocks distance of each other on both X and Z axis are almost always going to link to the same Nether realm portal on initial construction, unless more precisely paired portals are built (see below).