I agree with you and had the same question as yours before.
My son starts to say more now. One day I called my husband '老公' then my son followed and said '老公' (but he says '爸爸' normally). One day my brother called my mother '亞媽' then my son followed and said '媽媽' (but he says '婆婆' normally). Children like to follow and do/say what adult do. I think they just feel interesting and don't really understand the difference. Therefore, we start to call each one at the position of my son.