Because it's in the Developer Programme.
When you're enrolled in the programme you see the updates that are available in the programme. If you're currently on 13.0 and there's a 13.1 beta available that's what you'll be shown. The fact you're on 13.0 and there's an update to 13.0.1 is neither here nor there because 13.0.1 isn't in the Developer Programme.
Topic:
App & System Services
SubTopic:
Core OS
Tags: