..all of the above aside, the calculation looks correct if you ignore the quantity of the items and look at the individual items themselves.

1 item type grade 100
1 item type grade 100
1 item type grade 100
1 item type grade 31

Total types of items = 4
Total grades of item types = 331

331/4 = 82.75 rounded up becomes 83

So, can confirm what Elix said. Makes sense to me that it should work this way, considering that you can't mix item grades, it shouldn't matter what quantity of each item is required, just that there are x type of items being graded so they should sum as if the total item type quantity is 1