Missed chat tickets in Support don't show up that way for chats taken by agents. Why does there usually seem to be a discrepancy between these timestamps?
The timestamp relates to when the ticket is created. A ticket is normally created whenever an agent sends the first message in the chat. A missed chat, however, never had any agents that joined the chat, the ticket is only created after the chat has ended, hence the timestamp reflects the end of the chat rather than the first agent response.
For more information on missed chats, see the article: Setting up Zendesk Support in Chat.