Is british gas direct debit cheaper than pay as you go meters?

Im a new customer with british gas with dual fuel and customer service told me I will save 10% on my electric if I pay by dd (direct debit) fair enough. I have looked at british gas tarrifs and dd northern is 24p kwh and pay as you go card key meter is 18p kwh, In my flat I have a computer n lcd screen using upto 500wh fridge freezer about 50wh, hifi about 100wh, toaster, kettle about 100wh and 1 energy saving bulb at a time 11wh, these are the only electrical goods I have, they all use average 5-6 units a day = £8.50 - £10 a week, now ive asked a few of my mates who are all on pay as you go meters who have houses and they use between £10 - £15 a week with t.v.s electric cookers, dish washers, washing machines, tumble dryers, hair dryers and normal light bulbs. now if I had all them electricl items in my flat my electric bill would be sky high about £20 a week. So this is my question. why do I pay more with dd + 10% off than pay as you go?