Nope, you're reasoning is quite incorrect.
You don't want a spread in your AA wall, you want your target to die in 1 shot. Spread out AA can't reliably do that for many years now, you need multiple shell hits to kill a plane. Spread AA allows them to keep on flying cause they aren't even hit or just hit by 1 shell.
And that keep on firing after a plane is dead, is like due to the use of NF-fixer and some lag.