The nether portal takes you (randomly) to the same coordinates in the nether from the real world.. 

If I am at coordinates 0, 69, -200 in the over world and enter a portal, sometimes it will take me to nether coordinates instead of dividing them by 8... So I went in at 0,69, -200 and arrive at the exact same (more or less) coordinates in the nether... I also spawned without a nether portal (which was fixed upon relog)...