NSAttributedString boundingRectWithSize为表格单元格返回了错误的高度？

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
   NSAttributedString *aString = [self.rows objectAtIndex:indexPath.row];

   CGRect r = [aString boundingRectWithSize:CGSizeMake(self.tableView.bounds.size.width, CGFLOAT_MAX)
                                 options:NSStringDrawingUsesLineFragmentOrigin context:nil];

   r.size.height += 50.0f;
    return r.size.height;
}
- (CGFloat)tableView:(UITableView *)tableView h