The variation in the amount of bill could be because of the following reasons:
Change in Tariff - Tariff rates are generally revised annually.
Seasonal Variations - An unusual cold or warm weather can significantly alter your electricity use.
New electric appliances usage - If you have recently installed a new major electric appliance - such as air conditioner, refrigerator, washing machine or computer - your usage of electricity may increase.
Defective Electrical appliances - Defective appliances such as air conditioner, refrigerator, washing machine and geyser need to be repaired immediately as they consume more energy than normal.
More people in the house - More people in the house leads to more energy consumption and so a higher bill amount.
Changes in Lifestyle - The lifestyle is influenced by different work shifts, a new baby, illness, retirement, etc. These in turn affect various factors such as the amount of time spent at home, the number of times the door is opened/closed, the setting on the thermostat, time spent in watching TV, lights used, and number of showers, etc. All these result into higher amount of electricity consumption.