Why loading scene from AssetBundle is faster than loading from obb

Hi, we are trying to build our game to apk+obb include the game scene. We use SceneManager.LoadSceneAsync(sceneName) to load the game scene, and it spend about 6.6 seconds.

Then, we use AssetBundle to build the game scene bundle. And we build our game to apk+obb without the game scene in BuildSetting. We load game scene bundle first and call SceneManager.LoadSceneAsync(sceneName) to load the game scene, this way just spend about 3.6 seconds finishing the loading.

We Want to know why Unity load scene from AssetBundle is faster than loading from obb. Does anyone know how Unity read the scene asset from obb file? Maybe there are some additional work between reading obb file from disk and deserializing it to the game scene?

Other info:
Unity version: 2018.4.12f1
BuildAssetBundleOptions: ChunkBasedCompression
Test platform: Android