Hi Altamas!
Greetings of the day.

Firstly, I would like to know which software you are using.

secondly, This is very simple logic in general. If you select chain selection then it will cover the full outer edge. But for making this type of design simple disable chain selection and select the required edge and provide the required radius for that.

If you still have any other issues regarding this sheet metal design then just ping me in my inbox.
Thank you

Best regards
Azhar