Sorry, I´ve read those numbers as 4-21. My bad.

By aimaing and shooting (even if only grazing) the boss, the damage is already done. Normally that should be more fearsome for the assailants than actually killing the boss.
Vito and Gigante got away with it because after the Anastasia hit, Costello lacked muscle. He promptly stepped down and allowed Genovese to take over. The wast majority of the Commission members sided with Genovese.

In my opinion, the Costello hit was a botched hit. According to Valachi, Genovese took to his Atlantic Highlands home with some 40 men around him, guarding him. And key members of Tony Bender´s regime hit the mattresses. I believe these measures taken, after the Costello hit, would not had been necessary if Costello had been killed.
I think Genovese read Anastasia pretty good. I think most of the bosses did. In addition to his wild temper, Anastasia was also regarded as naive and one sided. As long as Costello was still in power (and still alive), he would have had taken up arms defending his close friend Costello. Not even Genovese, I believe, would have wanted to go against Anastasia. In these regards intentionally only grazing Costello and letting him live, would had been a stupid move.
Besides, why shoot at Costello´s head in order to just scare him off if that was Genovese´s plan all along? A shot to the leg would had been much safer. In my opinion.