The default instance is an arbitrary one in the project. Even if this was changed to something more specific like "the first instance placed in the editor", this too can easily change as you edit your project - e.g. imagine cloning the first placed instance a few times, then deleting a couple including the first placed one. Since you just deleted the default instance it will have to choose a different one, so everything changes again. So in general I don't think there's any way to choose a default instance that is immune to changing based on editing your project, which is why it's left as an arbitrary one.
The intent of the design is either they're all the same, so it doesn't matter, or you set the properties you care about by actions after creating the instance, in which case it doesn't matter what the starting properties were.