Why android Recyclerview automatically update without adapter notifyDataSetChanged() method called? when data is coming from web socket background

Question

I am showing list of data by recyclerview. so when data is comming from web socket then adding new items in existing list so my recyclerview update automatically. i am not explicitly calling notifyDataSetChanged() so how it's updating?.

If i will adding items by any button click event so it require to call notifyDataSetChanged() method then data will update but when data is coming web socket so it not require to call notifyDataSetChanged() method and data will update automatically.