Up until the 1948 Representation of the People Act, property owners could qualify for the vote in multiple locations. Frequently you will see a small business owner qualifying at his residence and at his business premises. At one time my grandfather was entitled to 5 votes due to his owning business premises, a farm and some woodland. In the case of your ancestor that may have been the answer, although I suspect that her being recorded at her parents' address was a mistake, or done earlier, perhaps when she and her husband were living with the parents or before she married, and the parents forgot take her name off the register.
The registers were compiled twice a year in the Autumn and Spring, and the compilation process took several months. Initially, forms would be sent out for householders to fill in and then there might be follow up visits by the Registration Officer's staff if necessary. But amendments could be submitted (say, by someone moving within the borough) right up to the cut off date (the so-called qualification date (1 June for the Autumn register and 1 December for the Spring one). During the compilation period any notifications of deaths were supposed to ensure that deceased voters were removed from the register, prior to the qualification date.
