Given a PageId, it returns the limits of the range of pages displayed.
While getOffsetByPageId() returns the offset of the data within the current page, this method returns the offsets of the page numbers interval. E.g., if you have pageId=5 and delta=2, it will return (3, 7). PageID of 9 would give you (4, 8). If the method is called without parameter, pageID is set to currentPage#.