That would explain if the order was rescinded, and the people following out disobeyed the top to call it off.

For Barzini though, why would he still go through with it? Barzini gets what he wants from Corleone, weakens Corleone's position, agrees to peace, and yet still goes through with trying to have Michael killed? To what end?

Seems like he had Don Vito right where he wanted him, and that it would unnecessarily rock the boat to go after Michael, especially after the "if he gets hit by a bolt of lightning" speech.