My best guess that Chrome works this way would be due to combining the sign-in cookies with the general cookies of the browser (e.g. 1 cookie store instead of 2).
So when you log in to Google, you log in to Chrome - and vice versa. For non-technical users this is a convenience feature, though for many it does come with privacy concerns.
Judging by the comments re cookie clearing, there is some part of the login still separate from that, but as it re-establishes itself the cookies come back and you stay logged-in to Google.
Is this perhaps preventable by having a "Guest" Chrome account? What happens then?