Think about it, maths and sciences for example, you could sort of guarentee yourself marks on a question or even near to full marks on the whole paper if you knew for a fact the stuff you were writing down what would give you full marks on that specific question because it's kinda formulaic in a way, but that's totally opposite for english as the subject is very free and with english you don't have an idea of what the marks you'll get be, but have a rough idea from mocks and reading paragraphs you've made. There is probably quite a few other factors though. To get full marks in english you'll probably have to be beyond PERFECTION but for e.g. a science you could just answer all the questions correctly to get full marks. Easier to pick up marks i guess aswell.