Randolph,
The user's Activity Log shows any action they've performed on different sites or on Facebook itself. What's shown on your Profile Timeline view is what Facebook thinks is the most relavent to the current viewer of the feed. The actions posted in your Activity Log, by default, may show up in many different places of Facebook, including your Profile / Feed View, your friends 'ticker' of what their friends are doing, and other areas.
When in the Activity Log, if you always want a certain action to show on your Feed, you can click the white bubble and say "Always show on Timeline". If it's at the default of "May show in Timeline", then Facebook's algorithms will decide whether to show the story based on the relevance of the action, if the viewer has the same app installed, how many users have performed the same action, etc.
Hope that helps explain,
Alex