What is it that divides grade 7/8 and 9 in english?

In previous literature mocks (year 10) i have received grade 9s for my english literature test , however as i resat the exact same test last November , i had received a grade 7 despite my further extensive revision and pre established knowledge. All that had changed however was who had marked my paper , previously in the year 10 mock it had been my teacher and via the year 11 mock , a randomised marker. Now this has raised the question for me which has gone indirectly answered by my teacher , what is it that differs a grade 7 and 9 response? upon looking at the mark scheme it speaks of ' judicious inference ' and ' developed arguments ' which in it of itself is very subjective to whoever marks , right ? the grade 7 requirement also consists of developed arguments and i struggle to see the lines of differentiation between the 2 grades. i am sitting my literature mock on the monday , so i wondered if anybody could give further explanation on how it is i can fully ensure a grade 9 , aha sorry this is so long.