Nether portals. Why do they get screwed up when I build multiple in the overworld?

Let’s say I build a portal in the overworld, then go into the nether, build one there and come back into the overworld through that portal. Everything works.
But if I build another portal in the overworld, and enter it or exit a portal in the nether, I end up popping out of a different portal than what it was connected to. Why, and can I avoid or keep this from happening. It would be nice to make multiple portals in the overworld. Thanks!