A bit of history might help here:
Most energy suppliers (both gas and electricity) used to charge the same price for each unit. However, because they have to supply and maintain the cables and pipes which go into your house, they added a 'standing charge' on top. (This was to cover the expenses that they had to meet even if you didn't use any power. It was equivalent to the 'line rental' element of a telephone bill). So, even if someone didn't use any gas or electricity at all, they would still have bills to pay because the standing charge was always present.
Consumer organisations (and groups who campaign on behalf of the poor or elderly) objected to this because, for example, it would mean that a pensioner who had to spend a great deal of time in hospital would still have to pay a disproportionately large bill for the very small amounts of gas and electricity which they used.
As a result of these pressures, the power companies changed to the present system. This means that anyone who doesn't use any gas or electricity (perhaps because they're in hospital) receives 'zero' bills. At the same time, it allows the power companies to effectively continue receiving the 'standing charge', from most of their customers, by spreading it across the first few hundred units used.
Chris