Tariffs assume a fuel cost for generating electricity. When the actual fuel cost for a month differs, the regulator approves an adjustment per unit, which is charged (or credited) in a later bill.
The adjustment is multiplied by the units of the month it refers to, so it can be large after a hot month. It is announced separately each month; Bill Ghar’s calculator lets you add it as Rs per unit.
Protected and lifeline consumers have often been exempted from some adjustments. Check the line on your own bill.