I don't know if I'd say, "I'm glad you asked." I'd feel akward, wondering if they felt like I had spent a lot of time preparing an explanation. I'd just be honest. If you've taken the time to think about why your score was low, and what you will change to stop that from happening again in a similar situation it shows you have learned from the experience and are moving on. I would suppose that is the most important point.
If you did poorly in the PS section and have good grades in chem and physics I would point that out. So you didn't ace the MCAT, your grades who you are able to comprehend the material.