I personally feel that LEA is the difference between passing and failing. Every other subject is either a small part of the exam or so random and specific (in terms of the questions being asked) that there doesn't seem to be a huge gap between schools that traditionally do well on boards and the national avg for each section.
When comparing our school's composite scores with national averages on each section, LEA was the one that jumped off the page (to me at least). That and it being worth 20% of the exam is how I came up with my little theory, for transparency's sake. It's more of a common sense reason than an empirical one but it seems to fit ok. Hopefully this guide helps many of the students who were lost on what to study and increases first time pass rate, but ultimately it is up to the students to study. Well, and the schools ought to be responsible for self evaluation in subjects where their students tend to be weaker, and fix those issues either with faculty/curriculum changes and/or promoting resources like this one to give students the best chance to succeed.
In all honesty, if I were to study for part I again I wouldn't bother even looking at this guide. But it certainly can't hurt to go over the topics that josebiwasabi mentioned