Why are a DisplayObject's child's .name property and the results of getChildByName() sometimes different?

var gfx:MovieClip = new a_Character(); //(a library object exported for Actionscript)

var do1:DisplayObject = null;
var do2:DisplayObject = null;

for( var i:int = 0 ; i < gfx.amSword.numChildren ; i++ )
{
    var child:DisplayObject = gfx.amSword.getChildAt(i);
    if( child.name == "amWeaponExchange" )  //An instance name set in the IDE
    {
        do2 = child;
    }
}

trace("do2:", do2 );

var do1:DisplayObject = gfx.amSword.getChildByName("amWeaponExchange");

do2: [object MovieClip]
ReferenceError: Error #1069: Property amWeaponExchange not found on builtin.as$0.MethodClosure and there is no default value.

var do1:DisplayObject = gfx.amSword.getChildByName["amWeaponExchange"];

ReferenceError: Error #1069: Property amWeaponExchange not found on builtin.as$0.MethodClosure and there is no default value.

var do1:DisplayObject = gfx.amSword.getChildByName("amWeaponExchange");