After answering a few short questions, often you'll be directed to more than one tariff.
These are shown by price but also named to identify the type of customer you think you'd fit best. For example, any tariffs stating "after 7pm" could be ideal for a customer who works during the day and uses most of their energy in the evening but this is not compulsory. You may still fit that profile even if some of the time of the year you are at home all through the day.
The reason for the slight changes is because we can buy energy at different times more cheaply than at other times so you are paying the right price for the energy you use!