When traveling through a nether portal from the overworld o the nether the coordinates in the nether are the exact coordinates of the overworld portal. Ex. If the overworld portal is at 1000 80 1000 then the nether portal will generateat 1000 80 1000. Then, when traveling through the portal from the nether to the overworld the coordinates are *8 as normal. In example above travel back through the portal puts you in the overworld at 8000 80 8000. Meaning it is impossible to get the original portal and you end up thousands of blocks away from the original portal.