Agatha was born in Torquay, Devon as 'Agatha Mary Clarissa Miller' in 1891. On the 1911 census she is 20 years old and appears to be living with her grandmother in Ealing, Middlesex, as well as living with her mother on the south coast in Torquay, about 210 miles away.
How can this happen ? I guess, as today, forms were sometimes completed prematurely and the individual, who had been expected to be at home, was for some reason away for the night. Another reason might have been that the parent didn't want to leave their child off the ten year census given it was just one night. They can't possibly have thought that over 100 years later their descendants would be picking up on their white lie !
Acknowledgement : Wikipedia - Agatha Christie as a child (age unknown)