That's a big gap and I don't think that exam technique explains this huge gap between your predicted grades and your achieved grades, but... yes, mark schemes are just incredibly pedantic. They were the crux of my existence in sixth form, particularly those for my science subjects because some questions, you could answer correctly without getting any single marks.
So if you haven't done so already, do more past paper questions and mark them yourself with the mark scheme. That way you'll familiarise yourself with what the mark schemes want.