Having been on the other side of the interview table for a few years now, I can tell you that board scores don't matter that much. If you've rotated with us, and we know you are a hard worker with aptitude, it easily trumps any low score. Also, if you do have a low Step 1, but you made an improvement for Step 2, that means a lot.
If we don't know you, then objective data becomes more important. Still, a good interview with good letters can outweigh a bad score.
I've only ever asked one person to justify their low score. That was because he spent time with us and I knew he was better than the barely-passing score indicated. So I wanted to give him the opportunity to explain to the attending sitting next to me. People should be prepared to discuss their deficiencies, but I typically don't point them out.
Hope this helps.