The box with the matching surnames on a DNA match's page is there to help you figure out the relationship. It doesn't mean that you are related to them through those surnames. Ancestry is showing you the surnames that you have in common with that match. It's up to you to examine their trees and find the connection.

As for the Unknown surname. A lot of people use Unknown for the surname when they don't know what the actual surname is. Others use ??? or _____ when they don't know the surname.