cesium cannot clustering CustomDataSource immediately when rendering?

When i replace the Sandcastle clustering data, using 1000 random points in CustomDataSource, the cesium cannot clustering CustomDataSource immediately, ONLY AFTER i move the earth a little bit. The Sandcastle clustering using the KmlDataSource. Is there any difference between CustomDataSource and KmlDataSource when rendering?
The code is as below.

var dataSourceForCluster = new Cesium.CustomDataSource('cluster');
var count = 1000;
for(var i = 0; i < count; ++i){
dataSourceForCluster.entities.add({
id: ""+i,
position : Cesium.Cartesian3.fromDegrees(Math.random()*360-180,Math.random()*180-90),
billboard :{
image : './img/junying.png',
scale : 0.25,
}
});
}
var options = {
camera : viewer.scene.camera,
canvas : viewer.scene.canvas
};

var dataSourcePromise = viewer.dataSources.add(dataSourceForCluster);

dataSourcePromise.then(function(dataSource){
var pixelRange = 15;
var minimumClusterSize = 3;
var enabled = true;

dataSource.clustering.enabled = enabled;
dataSource.clustering.pixelRange = pixelRange;
dataSource.clustering.minimumClusterSize = minimumClusterSize;

var removeListener;

var pinBuilder = new Cesium.PinBuilder();
var pin50 = pinBuilder.fromText('50+', Cesium.Color.RED, 48).toDataURL();
var pin40 = pinBuilder.fromText('40+', Cesium.Color.ORANGE, 48).toDataURL();
var pin30 = pinBuilder.fromText('30+', Cesium.Color.YELLOW, 48).toDataURL();
var pin20 = pinBuilder.fromText('20+', Cesium.Color.GREEN, 48).toDataURL();
var pin10 = pinBuilder.fromText('10+', Cesium.Color.BLUE, 48).toDataURL();

var singleDigitPins = new Array(8);
for (var i = 0; i < singleDigitPins.length; ++i) {
singleDigitPins[i] = pinBuilder.fromText('' + (i + 2), Cesium.Color.VIOLET, 48).toDataURL();
}

function customStyle(){
if (Cesium.defined(removeListener)){
removeListener();
removeListener = undefined;
} else {
removeListener = dataSource.clustering.clusterEvent.addEventListener(function(clusteredEntities, cluster) {
cluster.label.show = false;
cluster.billboard.show = true;
cluster.billboard.id = cluster.label.id;
cluster.billboard.verticalOrigin = Cesium.VerticalOrigin.BOTTOM;

if (clusteredEntities.length >= 50) {
cluster.billboard.image = pin50;
} else if (clusteredEntities.length >= 40) {
cluster.billboard.image = pin40;
} else if (clusteredEntities.length >= 30) {
cluster.billboard.image = pin30;
} else if (clusteredEntities.length >= 20) {
cluster.billboard.image = pin20;
} else if (clusteredEntities.length >= 10) {
cluster.billboard.image = pin10;
} else {
cluster.billboard.image = singleDigitPins[clusteredEntities.length - 2];
}
});
}

// force a re-cluster with the new styling
var pixelRange = dataSource.clustering.pixelRange;
dataSource.clustering.pixelRange = 0;
dataSource.clustering.pixelRange = pixelRange;
}

// start with custom style
customStyle();

var viewModel = {
pixelRange: pixelRange,
minimumClusterSize: minimumClusterSize
};
Cesium.knockout.track(viewModel);

var toolbar = document.getElementById('toolbar');
Cesium.knockout.applyBindings(viewModel, toolbar);

function subscribeParameter(name) {
Cesium.knockout.getObservable(viewModel, name).subscribe(
function(newValue) {
dataSource.clustering[name] = newValue;
}
);
}

subscribeParameter('pixelRange');
subscribeParameter('minimumClusterSize');

/*Sandcastle.addToggleButton('Enabled', true, function(checked) {
dataSource.clustering.enabled = checked;
});

Sandcastle.addToggleButton('Custom Styling', true, function(checked) {
customStyle();
});*/

var handler = new Cesium.ScreenSpaceEventHandler(viewer.scene.canvas);
handler.setInputAction(function(movement) {
var pickedLabel = viewer.scene.pick(movement.position);
if (Cesium.defined(pickedLabel)) {
var ids = pickedLabel.id;
if (Cesium.isArray(ids)) {
for (var i = 0; i < ids.length; ++i) {
ids[i].billboard.color = Cesium.Color.RED;
}
}
}
}, Cesium.ScreenSpaceEventType.LEFT_CLICK);
});

Start with the provided Sandcastle reproduction using 1,000 entities in a CustomDataSource, then compare its clustering behavior with the KmlDataSource example. Investigate why clustering appears only after camera movement; done means the CustomDataSource clusters immediately without requiring the earth to move.

Written by the indexing model from the issue text.