The orc may have needed another shot from Gimli's axe.... That butt-shot from Legolas was not his best shot to date... and not a fatal wound.
Legolas might still be trying to get the hang of shooting from a moving horse.
and not a fatal wound.... Easy for you to say...look at the location of the entrance wound again carefully...that Orc took it in the groin...probably severed his femural artery...not an immediate kill, but he'da bled out quick enuff...