While Wireshark is loading packets from a file each packet is dissected. Wireshark tries to detect the packet type and gets as much information from the packet as possible. In this run though, only the information shown in the packet list pane is needed.
As the user selects a specific packet in the packet list pane this packet will be dissected again. This time, Wireshark tries to get every single piece of information and put it into the packet details pane.