Google can use user prediction to guess which account you are with your browser, ip, ism, mac address, and other persistant metadata. Its how their whole 'you signed in from a new location' security scheme works.
Clear cookies, and you'll see it again.
In Chrome, it also uses the TLS Client Channel ID, which is a persistent unique identifier established between a browser and a server which (on capable platforms) is derived from a key stored in a hardware security module, making it hard to steal. Ie. if you clone the hard drive of a computer, when you use the clone, Google will know you are a suspicious person, even though you have all the right cookies.
Sounds like another reason to limit my personal use of Google's Chrome browser.