Within the documentation for Instance streaming, it is stated that the default behavior is LowMemory when it is actually Opportunistic. To reproduce, create a new Roblox place and check the streaming behavior.
If you create a new place using one of the templates the StreamOutBehavior will be initialized to Opportunistic:
If you actually set the mode to “Default” then it will behave the same as LowMemory. What the documentation is indicating is that the value of Default behaves the same as LowMemory.