I recently tried to restore a giant ruined nether portal and make it as my way to get into the nether. I removed all the crying obsidian lodged in the portal and replaced them with normal obsidian that I mined or found scattered around the portal. 

I completed my portal frame, lit it up, and came into the nether. I spawned in a Soul Sand Valley.

When I attempted to return back to the overworld, I was teleported to a completely different location. I was far from my giant nether portal. There was almost a 1000 block distance between where I was spawned and the original portal.

I've attached screenshots of my coordinates to give a better idea of what I am experiencing. you will see pictures of the ruined portal, my location spawn in the nether, and where I ended up going through the portal again.