Why Gmail Doesn't Show Unsubscribe Link at Top Always?

I am planning to integrate marketing newsletters for my users at www.hackpundit.com. Trying to understand few things: 1) Why unsubscribe link at top of Gmail is not shown for all emails even email has unsubscribe link at the bottom? 2) How unsubscribe link in top of emails affect the open, click rate of newsletters? 3) Is the unsubscribe link present only for marketing newsletters mails and not for transactional mails? Please suggest