If you look at a mortice lock key from the side (https://tradelocks.co.uk/cylinder-mortice-borg-locks/key-blanks/6-lever-mortice-key-blank.html), it is symmetrical from tip-to-base. This is because the same key can be inserted into the same lock from either side of the door.
however, on some doors with simple cylinder locks (like this) https://www.easykeys.com/381_Yale_Lock_1-1600_Lock_Keys.aspx the key can also be inserted from either side of the door, yet it is not symmetrical.
how does this work?