wrong registration of occupation

we bought a house and it was registered in august. we moved in September. we were paying pro-rata to the owner. he was paying the rent. he could not close his account as it has arrears which he did settle. allowing us to open our own account end of October. now the problem is the billing is from the date of registration 14 August 2015. and it does not refelect the two amounts the previous owner paid. I do not understand why must we pay money that was already paid.