What really happens is they keep you kind of logged in to collect as much data on your searches, videos, etc. The way to actually log out is:
- Click logout in Gmail
- Click "Sign in with a different account" (!)
- Click "Remove"
- Click the X next to your name.
- Click Done
It feels like they are trying to trick you into staying logged in by making the logout process extremely obscure and difficult, and I really hate it when I'm being blatantly tricked like that. With existence of evercookies (http://samy.pl/evercookie/) I am somewhat skeptical that even that obscure logout process actually logged me out.
This is pretty much the strongest reason why I don't use the GMail UI anymore (there are others, but this alone would have made me stop).