Super late response, sorry. As a low SES current applicant, the biggest difference, in my opinion, is concurrent financial responsibilities during MCAT prep. Working 40 hours a week over the summer, or 1-2 part time jobs during the school year leaves much less time to focus on test preparation. It doesn’t matter how great your resources are if you don’t have the time to benefit from those resources.
*Anecdote: I did fine on the MCAT, but I’m sure that number bumps up a bit if I had the ability to take 3 weeks off work and do full length practice tests.