this is an issue raised on many forums and it is a bit confusing.
There are two areas.
1. Any editable area is unique to that page and that page only so in a template no editable area will be updated that has to be done at page level.
2, Any "locked" area will be updated if you alter the template so you chage that and you get site wide updates.
Comes down to planning your editable and locked areas carefully - for my part I always get it wrong;-)