British Gas charges the first 223 KWh per month (for monthly billed customers) at a higher 'Tier 1' rate. All further consumption is at the lower 'Tier 2' rate.
However, because meter reading dates (whether actual or estimated) can vary by a few days, the figure of 223 units is moved up or down, pro rata. So if the meter reading dates were just over a month apart (probably by a day), the first 227 units would be charged at the higher rate.
Chris