I determined that the issue was due to me not having fiddled with the 'Letters' in the grading table. Turnitin passes a score out of 22 (dictated by me in the assignment settings) to the grading table, which then converts that to a percentage and then assigns a letter based on the letter boundaries in the grading table settings. 22 clearly does not go into 100% cleanly, so some students were still being given the wrong letter here simply because you can not have Letter boundaries of 4.54%-9.08%, they must be integers.
In the end my solution was to hide the grades in the grading table all together, meaning that student have to click through to the turnitin assignments before seeing their correct grade. This is not ideal but it is ok for this particular course.
Lessons Learned: In future, always have the number of grades in your grading scale a multiple of 100. In my case, the marker wouldnt even have to know this (I would have 3 empty or nonsense grades in the scale, and she could continue using my 1-22 scale as normal). If one trunitin assinment uses a different scale than the others, you cannot alter the letter boundaries for the grading table and therefore might aswell give up on marking turnitin assignments with letters.
Very frustrating, but at least students are no longer being given the wrong information, and I can forget about it until the next time this issue comes around!
Turnitin Direct V2 - Grades (letters) displaying differently in Moodle gradebook and Turnitin Assignment
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