Hi paulf,
Welcome to the AGL Community!
Winter in particular can see variances in the amount of energy used by storage hot water systems. This is because the starting temperature of the water as well as ambient air temperature is generally lower, meaning more electricity is required by your system to heat the water to its goal temperature.
It may be worth keeping an eye on your consumption as we move into the warmer months to see if this tapers off.
Kind regards,
Jayden