Direct dependencies of prefab variant differs from that of the original prefab

Use AssetDatabase.GetDependencies(prefabVariant, false) to get direct dependencies of a prefab variant, and the result is different from that of the original prefab when there are nested prefabs in the original prefab. It says that the prefab variant doesn't depend on the nested prefabs directly, but depends on the dependencies of the nested prefabs directly. Is this a bug?