Ask HN: How am I already logged into LinkedIn when I open an incognito window?

Often I want to look at a profile without showing that I've viewed the person, so I'll open an incognito window in a logged-out state even though I'm logged-in in a regular tab.

I noticed now I am logged-in when I open a new incognito window now! How is this possible? I can't recall any other website where this is true.