Similar story with my dd’s History result. She got a 4 in the mock, and her teacher said she is capable of 7-9 and she gets full marks in class work - crucially tho, that’s when she has access to the key dates and knowledge. Being really honest with herself, she did revise but did not revise ENOUGH to properly memorise the crucial knowledge for history.
Some insight I can share as a teacher myself (English) is that on Seneca, they really should be getting 100% for it to be meaningful. The way Seneca works is it shows you the correct answers for the bits you got wrong, then the idea is you retake those questions and get them right. That’s how it becomes a revision tool. So if he’s stopping at 75%, he’s not filling in his knowledge gaps. Also, check the data for how long he is spending on it. I often hear plaintive cries from my students of ‘but I did my Seneca! I got 60%’ or whatever, and I’m like yes love, but I can see that you spent 7 minutes 35 seconds on it, which is why we set 80% as a bare minimum 😆