My users tell me that not all @TaggedUsers are mentioned. For example, if a user taggs 10 other users in a post, maybe 3-4 of those 10 people will receive a notification. But all users have activated all notifications.
It's a user group setting which determines how many people can be tagged by the person making the post.
In your case it is set to 10 - so a maximum of 10 people will be alerted, regardless how many names are tagged.