Sounds like you bought Lion from the App Store, originally. It did not come pre-installed.
I suppose it's possible that the Store recognized your AppleID and that the previously install/purchased Lion was no longer on the machine you were logged in with. AFAIK, the Store doesn't track the machine you log in with, only your AppleID and your purchase history.
Secondarily, I
think the EULA says the OS is to be installed on
one machine. There are no "Family Plans" at the App Store.
