Why is hook_node_access($node, $op, $account) called with $op equal to "create"?
The admin menu contains links to create new content, and menu items also go through an access check to make sure the current user has access to the path. If they don't, the menu item is hidden.

The access callback for the path node/add/* is node_access(), which invokes hook_node_access(), which is why your access hook is being invoked when the admin menu is present.

You can either remove the admin menu altogether, or remove the links to create new content from it and your hook won't be invoked any more.