When Timeline was first being introduced and some people had Timeline and some people still had the old Wall format, if you deleted the story on your Wall about making a comment on a friend's post or photo, then later when you were converted to the Timeline format your Activity Log would not contain that activity. You MIGHT be able to find some of that activity by using the 'See Friendship' tool: point at the namelink of a friend on one of their posts, in the miniature 'Timeline view' window that opens there is a 'See Friendship' link. At best that is going to be tedious.