It’s not selected automatically “by accident/luck”. It’s inherited from the previous project you opened before you created this one.
Cubase works this way: If you connect any Audio Device for the very first time, Cubase tries to connect some ports. It tries to somehow follow the connection you had before with other connected device (for example if you were using port 3-4, it tries to use again ports 3-4). Of course, it might happen it doesn’t make sense for the new device, so the user has to adapt it.
Then Cubase follows this user decision.
When you create a new project, again, Cubase tries to guess what you want to do. And the most stable way seems to be just keep the I/O settings of the previous project (or make the “default” setup, you define by the “default” preset).
That’s the logic.