We treat each web browser on a single machine as a different device. Over the years, web browsers have increasingly sandboxed their data in order to protect privacy. Because of this, you can't share data, such as cookies, across browsers anymore. Also, with the multitude of devices that are identical from a hardware perspective (like millions of iPad Minis), using information at the system level which could be shared by devices looks the same. We only match devices when we are confident they are the same.
Think of this like logging into a bank's or some other secure website. If you change browsers, that site will not remember you were there, and you will have to login again. We don't want to accidentally "remember" you when you could be someone else.