Airtel — very high billing amount

Website:

airtel.in

I have one mobile connection of airtel (Postpaid) from j&k. My family is residing in bangalore. She also has a airtel connection (Prepaid). I got to know of the family connection plan of airtel and went to one of the airtel store. For this i had to transfer my connection to bangalore, and got one new sim which was activated in no time. They added my wife number as family number and gave me one more number as child number, which they told that it will be coming under my present plan (799* without taxes)
But now when i checked my bill it was unusually high, which when enquired they told that it is the previous balance amount. I dont understand this logic.
Please help me out