A calendar is essentially a sub-folder (if you will) of a mailbox that is set to contain appointment items, just as the Inbox is a folder that is set to contain mail and post items.
Basically, permissions are two-tiered. You can grant permissions to a mailbox, then you need to specify additional permissions at the folder level. There is no permissions inheritance.
<b>By default, nobody will be allowed to see the content of calendar items unless the mailbox owner has specifically granted access first to the mailbox and then additionally to the folder itself.</b>
So go and check the properties of the mailbox and make sure no one else has permsiion to the mailbox. Also make sure no one else has been delegated permission to the mailbox.