Why are some of my Contacts not listed - they are there when I conduct a Search, but are not in any of the lists.

Why are some of my Contacts not listed - they are there when I conduct a Search, but are not in any of the views such as Custom View or Business Cards. The Contacts not listed also appear in the “drop-down” when I click “To” on a new email. 

They are most probably in another folder.