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On the same machine, if I log in using the Linux side it says I'm logging in from a new location. Why?
Also, when I use Tor it says I'm logging in from a new location but that's expected because the whole point of Tor is the path is completely different.
There are many parameters which a server can use to identify a visitor. IP address is the first, cookies is another possibility, but the browser itself provides a "signature" including its full version, the name and version of the OS, the languages and encoding it supports... Also, it is easy to use JavaScript or Flash to identify plugins, screen resolution and other aspects of your machine configuration.
If you use all of it, it is too discriminant and the simplest browser upgrade will have you identified as "a new location". But choosing wisely an appropriate subset can give websites a good idea about whether you are using the same platform as before.
On Panopticlick, a web demonstrator of the Electronic Frontier Foundation, you can get an overview of all the information your browser is leaking, and to which extent it can be used to identify you.
