Google Apps [1] uses "cookies" to indicate that you are logged in. Occasionally, clicking on "logout" does not successfully delete the "cookie" that indicates that you are logged in, and returning to the Google Apps page restores your previous session. To solve this problem, click Logout, close all browser windows, and restart your browser.
Links:
[1] http://www.humboldt.edu/its/glossary/5#term40