Sitecore items inherit security settings by default. An item can inherit the access rights that have been specified for other items that are higher up the content tree. Any item can be configured to inherit the security settings of its parent item.
Sitecore allows you to configure which items should inherit security settings and which should not. The message you see means that someone configured explicit values for inheritance of the access rights for that particular item. It's not a bug.
You can read more about access rights and security in Sitecore Security Administrator´s Cookbook
The link is for Sitecore 6 but nothing has changed in terms of how Security works - only styling of Sitecore has changed.