affiliate@fulltilt.com is just the email address for the affiliate department at Full Tilt. They are the ones that manage all affiliates of Full Tilt, including TITN.
If you went to the Full Tilt website before downloading the software through TITN, you might have been tagged to a different affiliate not TITN. Even if it was weeks ago, if you clicked on a banner or link to Full Tilt, they put a cookie on your computer so that they know what affiliate sent you to them first. When you sign up the first affiliate that sent you to Full Tilt gets your account. Another reason could be if you have a "play money" account at full tilt, you can't open a real money account under a different affiliate.
To avoid this always clear out all cookies before you sign up for rakeback or for a promotion through a site like Poker Source Online. Just go to tools in your browser, Internet Options, then clear cookies.