Phillip Hallam-Baker and Roy Fielding are responsible for it. By the time they realized it was incorrect, too many people were using it.
Now, Phillip jokes about getting the Oxford Dictionary to recognize his spelling:
Its like when I did the referer field.
I got nothing but grief for my choice
of spelling. I am now attempting to
get the spelling corrected in the OED
since my spelling is used several
billion times a minute more than
theirs.
Roy also joked about the fact that the UNIX spell command didn’t recognize any spelling of it:
>Has anyone else noticed that the HTTP header “Referer:” is spelled
wrong?That’s okay, neither one (referer or
referrer) is understood by “spell”
anyway. I say we should just blame it
on France. 😉
Info taken from HTTP_REFERER Origins Wikipedia article.