They have given you the credit of the 3 month advance you have paid. That is not the issue.
The issue is of effective start date of your connection. You claim that it started from 2nd April. Their bill indicate that it has been considered to have started from 27th March. And the amount charged is the rental difference of these 6 days from 27th March to 1st April.
So you have to take up with them regarding the effective start date. If they revised it to 2nd April, the amount charged will get automatically reversed.
The issue is of effective start date of your connection. You claim that it started from 2nd April. Their bill indicate that it has been considered to have started from 27th March. And the amount charged is the rental difference of these 6 days from 27th March to 1st April.
So you have to take up with them regarding the effective start date. If they revised it to 2nd April, the amount charged will get automatically reversed.