Cashback site different from actual cost

Pound

I switched my power to Npower on the energy helpline site. I chose Sign On-Line 6 tarriff, charge is 3.821p for the first 1143 kWh and 2.149p thereafter.

Got a letter from Npower telling me they charge 4.042p or the first 4572 and 2.177p thereafter.

Clearly a big difference. Npower have said the price on the cashback site is wrong. What has happened here? How can the adverised price be so different?

tripled

According to their website the rates for gas are 3.639p per kWh for the first tier, additional kWh per year 2.047p per kWh (+vat @5%), matching what the switching site says. Are you sure nPower have you registered on the sign on line tariff?

All nPower's prices are here: http://www.npower.com/at_home/electricity_and_gas/prices_and_discounts/prices_across_uk.html

Pound

The confirmation from the cashback site showed the tariff as sign online. Npower's letter doesn't show a tariff but as far as I can tell from the prices it looks like I've been put on the standard tariff.

On the letter I've noticed it says direct debit details must be provided on making the application to be elligible. I opted to have a form sent out so I could fill it in and send it back. Could this be the reason I'm charged higher rates?