Nether Portal Mismatch

I've created a nether portal very, very, very far away from all other portals. It transports to the nether just fine, cool, and going back...it can't stick the landing. It keeps creating a new portal somewhere totally different in the overworld. I can't figure out why.

 

Starting overworld coordinates: 

7034.10, 71.06, -6008.15

Nether portal coordinates:

850.50, 34.51, -673.50

Current landing point back in overworld:

6811.37, 94.55, -5380.92

 

Any ideas? Is this just a lovely glitch that I've discovered?