Why taaya and chaacha?

Mother's sister is maasi. Mother's brother, maama. Father's sister is bua. But father's brother? He could be chaacha or taaya. I found it interesting to note that father's brother is the only relationship in which the age-based heirarchy matters. This becomes even more curious when we consider that the father himself uses only one word- bhaai- for all his brothers. He has to use the adjective vadda/bada and chhota in order to tell whether a particular brother is elder to or younger than him. Why then do his children use the distinct words for his elder and younger brothers?