Our definition of 'device' means a designated unique device_id property and does not mean a single physical device! it probably shouldn't be called 'device' at all!
This is a unique ID that is shared in the browser local storage (for web). So, if you use different web browsers or incognito mode of the same browser, then these will register with different device_ids and therefore count as different 'devices' that will show up separately in your device listing dashboard.