When an energy tariff expires, energy companies tend to "roll their customers over" to a standard variable rate for electricity or gas. This is a very expensive rate in comparison to any fixed tariff, but most people never end up switching to a better deal when this happens.
Ofgem (the Office of Gas and Electricity Markets) recognises that most people don't switch themselves and end up stuck paying too much for their energy. Because of this, switching service providers are required to take this "rollover period" into account when they make savings calculations.
For example: if you have 8 months remaining on your current tariff, there would be 4 months of "rollover" in your savings calculation, at a higher rate.
For people who are very on the ball with switching their own tariffs, this can come as a surprise, but for the vast majority of our customers, it's much more accurate to calculate how much they'll save this way.