The number of units charged at the initial higher price is always scaled to the time period.
Although I pay for my electricity weekly, I still get quarterly statements. However the date between the meter readings is never exactly three months. If the period is shorter, the number of units charged at the higher price is reduced. If the period is longer than three months, more units are charged at the higher price.
Although you pay monthly, I doubt that your bills are actually calculated monthly. They're probably calculated (as mine are) on a nominal three month basis, with your total payments being deducted from whatever you owe.
Chris