i feel like to get a 7, you can have an alright essay with a few nibbly problems and nothing amazing, or some really good components with a few others that are average, whereas once you hit the 8 region, the essay is generally quite good, and it is just lacking in a couple things eg: sentence structure, poor flow of writing, maybe bad examples, maybe not too good of an idea/contention but it was executed very well (this last one sounds more like a 7 after I've typed it out). Most 9's are good essays, with good flow and use of the language, and a good idea, but quite often you'll get a 9 because there will just be a few little things that arent perfect, or it has no zing (not saying you need an out there idea or essay), but you can read a 10/10 essay on a boring topic with a boring contention, and use BORING language thats only purpose is to tick the boxes, and you read it and you just say "there is absolutely 0 wrong with this essay, it is written well" and though it might be a boring essay, it could be written in the way that makes it as least boring as possible, but even then you could get a 'it just didnt grip me' 9.
my schools teachers feedback last year (select entry school) when i was with them and they were showing me 10/10 essays and telling me why i didnt get 10/10, was that 10/10 usually just grab the examiner. and its a whole bunch of factors, alot of it is what ned nerb said where the writing and the analysis is on point. the sentences flow nicely, there is no weird punctuation or grammar or long windedness, and i have definitely seen 10/10 essays where structure was sacrificed in some areas because the idea had absolute flair and the writing is so eloquent that succumbing to the structural conventions for the subject would take away from the pep of the essay. i feel like when people write alot of 9/10 essays for practice, eventually they will write a 10/10, read it then re-read over the 9/10s, and theyll just know its a better essay, even if they cant put their finger on it. its not even the vocab that sets you apart in terms of the writing, some essays are just written in a way that flows and is eloquent, and the ideas are so good that its got the examiner reading it and nodding their head thinking "damn this student is slick". big words that are too big and i dare say rarely used often take away from the essay cos it gets people thinking about the word rather than the idea. its usually a 10/10 essay that will correctly use the word 'loquacious' which any english teacher will know, and a poorer scoring essay that will describe a character as 'garrulous' which the student learnt off their 'learn 1 word a day at dictionary.com' subscription, and will get teachers scratching their head thinking "lmao this dude has grammar and flow errors, i doubt he even knows how to use that word"