You stated earlier....
"But CNX Immigration charges 1900 baht for the second one and only 1 week."
There is only one 30 day extension for tourist visa entry.
The second application resulting in 7 days is not an extension, rather 7 days to get out of dodge.
The reason immigration processed your application is that folk that require few extra days (less than 7) can apply knowing full well the application will be rejected and as a consequence be given 7 days to exit Thailand.