Because election commision of India has the correct information of number of eligible voters in the state in every ward. When a voter comes to station, prooves his identity, he is allowed to cast his vote. His excercise of ballot is cross checked against the his identity. When this exercise is pooled together across all the wards in the state, the actual number is of voter turnout is estimates. As EC is not regulated by any political party and is an autonomous body, (more than even supreme court of your country at multiple instances. ) It would be safe to attach a very high degree of reliability with the election commission of India's published numbers.
I am not sure which additional body would be capable of giving out better voter turnout ... if it is areesh and co, then you are more than welcome to conduct your own surveys, employing which ever statistical means your want to employ. Just a plea would be to not publish your stats by counting people from pictures you posted as the alternate credible source!