I'm not making adjustments to the prefabs orientation when spawning. The orientation is exactly what it was when the prefab was first saved by the author.
It you happened to be in another location on the same planet, it will still have the same global orientation, but appear different because of where you are on the sphere.

That said, the /addprefab command wasn't intended to perfectly place a ship/station. Just a quick method to spawn a test craft without fiddling with the SpaceMaster and blueprint menus, and to also check that the any prefabs you are trying to create for a mod, are working as intentded when spawned.

If you are trying to place an alien station, your best bet would be to:
1. use spectate into space.
2. spawn the chosen prefab.
3. Cut (Ctrl X) the station.
4. spectate back to the chosen location on planet.
5. Paste (Ctrl V).... rotate for you chosen orientation.
6. Confirm (left mouse click) paste.

Ok thanks for your reply!
That makes sense, I will try that out! :)