Keep in mind, I've never met her before
Most Helpful Girl
It could be as some people are saying that she was blushing from the remark. But honestly, the first thing I thought of and I think is the most likely scenario, is that it's an instinctual gesture of humility. That slight decline in the head shows that she is humble and is accepting of the compliment but doesn't let it inflate her head. I think a lot of people when accepting something humbly slight bow their heads when receiving it. It's an instinctual body language similiar to standing tall with shoulders pulled back shows confidence.