When you enter a Nether Portal in Survival mode it teleports you in the other dimension to the exact same coordinates of the portal you got in (without creating a new portal, it just teleports you and not respecting the 1-8 scale Nether-Overworld).

e.g. You have a portal in the overworld at 1600/10/800. When you enter it in Survival mode (It doesn't work if you're in Creative) you'll get teleported to 1600/10/800 in the Nether, even if you have the right portal at 200/10/100 in the Nether.