The reason for this is that you've launched an ephemeral instance and ephemeral storage doesn't count against your block storage usage.
An ephemeral instance means that everything involved with the instance is temporary, including the hard drive. When you terminate the instance, it's gone forever.
When you boot an instance from a volume or create a volume, that volume can stick around even after you've terminated the instance it's connected to. That allows you to keep data on that volume and use it later. Think of it like an external USB drive for all your DreamCompute instances.
Only when you create a volume (i.e. hard drive), does that count against your block storage allowance.
Hopefully that clarifies things a bit!