If your Viber was reinstalled in the past, it's possible that the messages were kept as a backup and when the new installation took place, Viber restored those messages. You can't have contacts on your Viber if you don't add them to the contact list of your phone. This is the way Viber works with contacts. It's possible to view past conversations if the message history is saved as a backup. Viber might recognize the history and add it to the message list.