That's just how the settings work. I doubt there was a database entry at all. When you call qa_opt(), it returns a NULL value when the setting doesn't exist (unless there is a default set, which there isn't in this case). For PHP tests, NULL is effectively the same as 0, so this is fine. If you do a test:
you'll get false either way. If you check the checkbox, it will be saved as a 1 in the database, and then the above test will return true. If you uncheck it later, it will be saved as a 0, effectively the same as not having the setting at all. That's just how the script works.