Ah ok, that sounds like correct behavior then. In PHP, sessions aren't tied to a browser window/tab, they're tied to a cookie which is tied to the whole browser. So you'll find with the vast majority of PHP scripts (including Centova Cast's web interface) that if you for example login, then open two tabs, then logout from one tab, you'll find that the other tab also shows you as logged out the next time you do anything in that tab.
When you login to a client's account in Centova Cast (from the account list, using the "lightning bolt" icon), you are completely changing your user context from "admin" to "user". So anything you do in ANY tab is now in the context of that user.
When you logout from ANY tab, you will be returned to the admin account.
And -- for convenience purposes -- if you attempt to access any admin page after logging in as a user from the accounts list, it will automatically log you out of the user account and back into the admin account, just as if you'd hit the logout link yourself.