Visa rejected because I am the only child

[moon]

My parents went to Chennai consulate for visitor visa. They got
rejected, I think it is because I am the only child. The officer asked 'How
many sons and daughters do you have?' My father said only one daughter. He was very surprised to hear that (I dont know why) and he stressed saying
'Only one daughter?'. What are your suggestions. I am planning to take
another appt. with a gap of 4 months. And I have greencard, may be that is
a cause too.

If anyone had similar experience, please suggest what to do next. Any help is highly appreciated.

Thanks a lot.