PHP apache_request_headers() diagrees with reality (as confirmed by Firebug): why?

'Host' => string 'peterh.labs.example.com' (length=26)
'User-Agent' => string 'Mozilla/5.0 (X11; U; Linux i686; en-GB; rv:1.9.0.3) Gecko/2008101315 Ubuntu/8.10 (intrepid) Firefox/3.0.3' (length=105)
'Accept' => string 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8' (length=63)
'Accept-Language' => string 'en-gb,en;q=0.5' (length=14)
'Accept-Encoding' => string 'gzip,deflate' (length=12)
'Accept-Charset' => string 'ISO-8859-1,utf-8;q=0.7,*;q=0.7' (length=30)
'Keep-Alive' => string '300' (length=3)
'Connection' => string 'keep-alive' (length=10)
'Referer' => string 'http://peterh.labs.example.com/qmail/' (length=40)
'Cookie' => string 'WORKFLOW_SESSION=55f9aff2051746851de453c1f776ad10745354f6' (length=57)
'Pragma' => string 'no-cache' (length=8)
'Cache-Control' => string 'no-cache' (length=8)

Request Headers:
Host             peterh.labs.example.com
User-Agent       Mozilla/5.0 (X11; U; Linux i686; en-GB; rv:1.9.0.3) Gecko/2008101315 Ubuntu/8.10 (intrepid) Firefox/3.0.3
Accept           text/html,application/xhtml+xml,application/xml;q=0.9,**;q=0.8
Accept-Language  en-gb,en;q=0.5
Accept-Encoding  gzip,deflate
Accept-Charset   ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive       300
Connection       keep-alive
X-Requested-With XMLHttpRequest
Referer          http://peterh.labs.example.com/qmail/
Cookie           WORKFLOW_SESSION=55f9aff2051746851de453c1f776ad10745354f6