Quiz

Hello Tim,
I have a problem about Restrict Access function in Moodle. We are using many restrictions in our course based on comletion and grades and I am trying to tabulate restriction structure for previous courses.

Within Moodle, when I edit an activity and add restrictions for Grade and Completion. I can see the id`s of both items (the quiz whose access is restricted(mod/quiz/view.php?id=39438) and the prerequisite item (mod/quiz/view.php?id=38594))
When I check the mdl_course_modules table and investigate the availability column, for the completion restriction everything is clear, I can see the restriction from the query:
id availability
39438 {"op":"&","c":[{"type":"completion","cm":38594,"e":1}],"showc":[true]}

But when I add a restriction on grade to be minimum 75 for the same quiz:
id availability
39438 {"op":"&","c":[{"type":"grade","id":7981,"min":75}],"showc":[true]}

I couldn`t understand the meaning of id:7981,
When I check, mdl_course_modules table, I find the instance as 6684

From mdl_quiz table, when I check for id 6684, I see the correct quiz, not a quiz with ID 7981.