Nifty vs Sensex Correlation Magic

I downloaded the daily returns from the both the indices since 1st Jan 2017 to till date
What I observed is that the daily returns given by both the indices are more or less same and only on 5 occassions out of 104 total trade days, there is minor deviation of 0.2%. What I am interested in knowing that inspite of Nifty being 51 stocks & Sensex being 30 stocks, how can the returns be so very correlated ? Expert Views
Is index value managed by some force or what is that clings both the indices so much to each other ?? :scream:
Attached screen shots for the debate