We changed to twice yearly statements as it makes it easier for us to accurately review your account and payments. People use different amounts of energy throughout the different seasons - you may notice that your account will tend to go from being in debt (when you're using more energy) to being in credit (when you're using less).
We even out these 'peaks' at the end of the year so your account shows a zero balance. If we took a look at your account at various times throughout the year, the information we would see could look misleading and cause confusion.