As Data10 notes, this is related to essay content and topics, not rating by admissions counselors.
Why does “easily countable features of essays, like the number of commas they contain” mean “correlation with wealth” is “even worse” and "$150/hr, can help turn a mediocre essay into a very good one. "
From my read, the conclusion is that students in different SES groups write about different topics, using different words.