Robbie the MCAT is just 1 aspect of your application. That is interesting that there is a "discrepancy" section of a secondary. I don't recall these when I applied.
I suggest a thoughtful answer reflecting on what went right the first time and what could be improved upon your second attempt. Avoid a long explanation, and own it. Medicine is all about self reflection and learning, and this may be an opportunity for you to illustrate it - as frustrating as your result may be.
Of course, there is always the viewpoint to abstain from illuminating things which may not need to be. Only you can make the decision on that.
And yes, I agree. A 507-503 is certainly not a death sentence as the previous poster stated.
Best of luck.