Do you have another Android device (like a phone, maybe) that you'd previously connected to that same wifi point?

Android 4, Ice Cream Sandwich, (and possibly older versions too, though I don't have any to hand to check) has an option to sync saved wifi passwords between different Android devices on the same Google account. Here's the option under **Settings** -> **Backup & Reset** on my phone.

![ICS Backup & Reset settings screen][1]

There are also apps that allow you to s[ync wifi passwords between your laptop and your Android devices][2], though I assume you'd know if you had one of those installed.