Mar 4th 2014, 18:15:46
Both OS updates and Browser updates cause the EE servers to think you are on a new device (because all major browsers report both the client browser version and the OS version to the server in the HTTP request header).
This allows websites to target the user based on the browser he is using, and/or the OS the user is running, by showing the user the correct page (for eg, Mac downloads vs PC downloads, or mobile version vs Desktop version), and also for websites to do demographics, as well as identify issues that only certain browsers/OSes encounter.
This week has quite a number of browser updates as well as OS ones. Even MacOS got a security patch.