Know your address

Sometimes my Gmail account gets emails to it that don’t belong to me. These are usually legitimate emails intended for some probably-distant relation with my last name but a different first name. I can understand that, because it just means that the sender got the address wrong when they sent the email.

However, recently I got a confirmation email for an online purchase under someone else’s name. One of these rogue Casteels purchased something from American Eagle and then put in the wrong email address when they checked out. How do you do that? Don’t you know your own email address?