"The menu text varies. It just does; why ask why..."
I think it changes based on your cached history of the last 30 sites you've visited, which Amazon pulls from your browser. For example, if you visit neimanmarcus's or tiffany's site, and then Amazon, the "Log Out" changes to "Leave this low rent jive store."
I could not find the "sign out" button either. It use to be right under "your account". I searched for a while until I found it under "help". Guess they don't want you to leave the site until you purchase something ;)