Locked HD

With a new MacBook Pro, I have just cleaned up my old MacBook Air to give to a friend. It wasn't easy and took several attempts to erase the partitions and return the laptop to its pristine state. But at last I saw the familiar invitation to start afresh with the first question being which country I was on.

But apparently it wasn't that simple. When I took the Air to the friend and starting helping to install, it suddenly asked for me to log in with my own ID, which should have been completely removed.

What did I do wrong? And how to solve without starting from scratch again?