Not showing user names on folder permissions when users look to see who has access

Hi,

    I have a customer with very strict security settings on the folders within there network drives. I have set this up by going into the security settings via the properties of the folders.

When I look at the security settings I can see the user names, However when a user looks they can only see a series of numbers and letters (see attached image).

Any ideas as to why users cannot see who has access?

Thanks
Security-Screen-shot.jpg