Portal linking issues

I created a portal at my spawn area, connected it to the nether, no problems. I then in a different session, i travelled Overground 500-1000 blocks away and created another portal, when I entered it it took me back to my original portal in the nether, I can say I was fairly confused at the time. When I went back through to the nether I was in the original nether location again and then back to spawn again. Is this a glitch or do overworld portals need to be further away from each other? They didn't used to have to be.