Hey @Ashish_Sahni
I was wondering this very same thing when I upgraded my M2 cluster to M10. Plus the some 30 connections.
The reason I received was the intercommunication between your nodes which make up your cluster. They are in ‘constant’ contact with each other. Plus Atlas provides monitoring of your cluster which also required data transfer. At the end of the month I believe it was around 2% of the total bill.
The documentation mentions this about data transfer
The vast majority of Atlas customers spend less than 10% of their budget on data transfer.
Hope this helps.