Why do you think Kisa Gotami understood this only the second time? In what way did the Buddha change her understanding?

Kisa Gotami understood the truth about death because the first time she was overwhelmed by the feeling of loss due to the death of her son. She was not able to see the bigger truth behind the actual situation. Buddha helped her in understanding by sending her house to house to find the house where nobody has ever dies. This expanded her viewpoint and she was able to see beyond her grief.