Nether portals not linking up properly

I have just started a new survival world, Normal mode, no cheats.

I am building my base around spawn area and have just built a nether portal.

Whenever I go to the nether it goes to the same originally generated portal but when I come back it does not come back to the same portal I made.  I have tried this 3 times now breaking the automatically generated portal in the above world between each attempt and it generates a different portal each time a few blocks away from the previous one.  These are approximately 140 blocks away from my original portal I created to get to the nether in the first place!