BuildAssetBundle the same prefab couple times,but the target files are not the same(md5)

i create a nearly empty prefab,and i BuildAssetBundle it couple times,the target files are the same(md5). that's good,but what happend next is a nightmare!! i close the unity,then open the unity,then BuildAssetBundle just the same prefab,then the target file changed!!! WTF!! i have tried many times,and i compare the difference,i can't tell why this happen.

BuildAssetBundleOptions code:

 BuildAssetBundleOptions bop = BuildAssetBundleOptions.CollectDependencies | BuildAssetBundleOptions.DeterministicAssetBundle;
 BuildOptions bo = BuildOptions.UncompressedAssetBundle;