Maybe because "the big six" main suppliers all put their prices up and the company I use didnt!!! It isnt rocket science.
To be fair (and honest) with you, part of that saving is down to them be a fair bit cheaper and partly to do with the cashback I get on all my daily expenditure. I save on average a further 5% on all my shopping at ASDA, Sainsbury's, M&S as well as petrol purchases, meals out to a few restaurants. It all adds up. I probably get £30-40 per month knocked off my bill just from the cashback so I think its a good deal.
At the end of the day, they GUARANTEE to be cheaper than all the other companies and if they are not they promise to give you back double the difference. I was sceptical too when I first found them but having been a customer for around 18 months I can safely say, they are a good company.