Why default template suggestion use double dash instead of single dash?
I do believe it's just to add separation on the off-chance that the region/content type/page name has a dash in it, itself. block--my-impressive-module--delta.tpl.php is a lot easier to see that the intended target is block named my-impressive-module, than if it were block-my-impressive-module-delta.tpl.php (now there's no separation between the three name sections). It's possible for a region/content type/page name to have a dash, but none will have two dashes, so it's a reasonable substitution.