It is due to the magic of the client. If you only type the first name, the client will look up the character in the Awake , Watch, or In Room lists, depending on command.
- whispers looks in the In Room list
- messages looks in the Awake list
- mail looks in Watch and Awake list.´
If only one matching character is found, it assumes that one.
Also see topic: [Documentation] Clarify single name vs full name - #3 by maximus
/Accipiter