One Part One Site, One Site One SelectionProvider, When you switch between
difference views or editors, the Part changed, so the SelectionProvider
changed also.
In WorkbenchPage class:
private PageSelectionService selectionService = new
PageSelectionService(this);
private WorkbenchPagePartList partList = new
WorkbenchPagePartList(selectionService); // selectionService construct
relation with partList

method: private void setActivePart(IWorkbenchPart newPart)
…
partList.setActivePart(partref); //when select difference parts, it will
update selectionService, at some time, PropertySheet will update too.

In WorkbenchPagePartList class, fireActivePartChanged method will do the
real things:
protected void fireActivePartChanged(IWorkbenchPartReference oldRef,
IWorkbenchPartReference newRef) {
partService.setActivePart(newRef);

IWorkbenchPart realPart = newRef == null? null : newRef.getPart(false);
selectionService.setActivePart(realPart); //this also changed selectionService
in WorkbenchPage class.
}

Last, the selectionService in WorkbenchPage and selectionService in partList
is same .

http://www.eclipse.org/articles/Article-WorkbenchSelections/article.html