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.