I'm thinking RowNum 1 may not have a date between your selections. Maybe put the date selection inside so that you know that the first one matches.
SELECT Referer
FROM (SELECT ROW_NUMBER() OVER (ORDER BY CT.Referer ASC)
AS RowNum, CT.Referer, CT.LastModified
FROM ClickTrack CT
JOIN OrderTrack OT ON OT.ClickTrackID = CT.ClickTrackID
WHERE CT.LastModified BETWEEN '07/06/2013' and '08/05/2013'
GROUP BY CT.Referer, CT.LastModified
HAVING LEN(CT.Referer) > 0) as num
WHERE RowNum = 1