Spawn object And assetId

I have a problem. In my code i use the the function ClientScene.RegisterSpawnHandler(currentAssetID, SpawnGo, UnSpawnGo); to register custom object at client, so i need the custom object assetid, i use the www to download this object from other server the code like this

then i debug the item.GetComponent().assetId; this assetid=00000000000000; but i upload the AssetBundle ,the assetid=44a4a57ad1e78c64bb87ecabacf54692 I don't know why two times ,this assetid are not equal. who can tell me why? thx.