Douglas Adams förklarade faktiskt detta på sidan 50 i boken för radioskript .
Many people have asked me angrily why it is that Zaphod Beeblebrox instantly greets Ford as Ford when I had earlier stated quite clearly that he had only changed his name to Ford Prefect when he came to Earth.
It was very simple. Just before arriving he registered his new name officially at the Galactic Nomenclaturoid Office, where they had the technology to unpick his old name from the fabric of space/time and thread the new one in its place, so that to all intents and purposes his name always had been and always would be Ford Prefect. I included a footnote explaining this in the first Hitch-Hiker book, but it was cut because it was so dull.
Personligen föredrar jag min egen idé, vilket är att det bara var en artefakt av Babel-fiskoversättningen.