Ask HN: how did my Nexus 7 know my wifi password?

I got my Nexus 7 delivered to me at work today. I got to play around with it a little bit during the day, but didn't really do much except login to my work's WiFi and my Google account.

When I came home, I knew I had to look for the password to my WiFi (random string of numbers so never remember it). To my surprise, when I woke up my Nexus 7 it was already connected.

My WiFi still needs a password and nothing has changed to it since I hooked it up a year ago. How did this already know my password?