There is a basic concept in asp.net called "Sessions". I suggest you read about it if you haven't. Usually a single session is maintained across all tabs in a single browser instance. Nowadays all browsers support tab based browsing, and almost all browsers have been designed to have a single session across multiple tabs for the same browser instance - it has been designed that way for specific reasons which has its own share of pros and cons. Have a look at this link to understand more about it.
http://geekswithblogs.net/ranganh/archive/2009/04/17/asp.net-session-state-shared-between-ie-tabs-and-ie8.aspx[
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You try the same exercise by opening a totally different browser instance of Mozilla, it will surely re-direct to the login page.