UnloadUnusedAssets can't unload a asset?

HI Guys i use AssetBundle.LoadAllAssetsAsync api to get my Asset,but when i remove it,it always in memroy. Then i use LoadAllAssets api and UnloadUnusedAssets work.
我获取了一个ab包后，使用LoadAllAssetsAsync 接口来获取Asset对象。但是当我清除所有引用并调用UnloadUnusedAssets后在profile工具中还能看到内存占用。如果换成LoadAllAssets 接口获取Asset对象就能正常释放了。请问可能是什么原因？

ps: by unity5.6.5 and test in phone
here is my code:

Code (CSharp):

1.  public class test : MonoBehaviour {
2.      public AssetBundle temp = null;
3.      public AssetBundleRequest req;
4.      void Start () {
5.          temp = AssetBundle.LoadFromFile(Path.Combine(Application.streamingAssetsPath, "uialtas/340011.pkg"));
6.          req = temp.LoadAllAssetsAsync(typeof(Sprite));
7.          //temp.LoadAllAssets(typeof(Sprite));
8.          //DestroyImmediate(img.gameObject);
9.          //temp.Unload(false);
10.          //StartCoroutine(OnExcuteUnloadUnusedAssets());
11.          //Resources.UnloadUnusedAssets();
12.      }
13.      public void Clean() {
14.          Resources.UnloadUnusedAssets();
15.      }
16.      public IEnumerator OnExcuteUnloadUnusedAssets()
17.      {
18.          yield return Resources.UnloadUnusedAssets();
19.          //Debug.Log(1);
20.      }
21.      // Update is called once per frame
22.      void Update () {
23.          //if (req!=null&&req.isDone)
24.          if(req!=null&& req.isDone)
25.          {
26.              req = null;
27.              //    Resources.UnloadUnusedAssets();
28.              //UnityEditor.EditorUtility.UnloadUnusedAssetsImmediate();
29.              if(temp!= null)
30.                  temp.Unload(false);
31.              StartCoroutine(OnExcuteUnloadUnusedAssets());
32.              //Resources.UnloadUnusedAssets();
33.          }
34.        
35.      }
36. }

here is my test project