Because they are smarter than the average adult. Strange things do happen. I remember a family, mother was English, father was Swiss and spoke fluent English, but with an accent. The children spoke Swiss German and English. However, they spoke English like Daddy, and not like Mummy.
I also know a Turkish/Kurdish family who always speak Turkish with their children. I noticed that, if the parents wanted to say something which the children shouldn't understand, they spoke Kurdish. One day I said to the father, "Be careful, A , (the oldest child) is smart, She already speaks Turkish and German and she'll soon start to understand Kurdish." Barely 3 weeks later, he said to me, "You were right. The other day I said something to my wife in Kurdish and A said, 'OOH!' When I asked her if she had understood, she translated what I had said into Turkish."