Precious word 'Mum' from my boy

Today is a special day for my wife as for the very first time, my boy, after more than 15th months, has began to call his mother 'Mum'. What I don't really understand is that the word 'Mum' only came out from his mouth after he knew how to pronounce 1-10, A-Z in sequence and the name of over 100 objects shown to him using flash cards.