I don't think it is a mistake, I think it's more likely a lack of credit history. I was the same, no debts, always paid my bills on time and had a credit card but rarely used it. So effectively you have very little credit history so they don't know how good you are at paying back money. I had to pay £150 deposit to get my iPhone with o2 which was refundable after 3 months, if I were you I'd get a free credit check just to make sure and then start using your credit card on a regular basis. I use mine instead of my debit card and just make sure it gets paid off before the end of the month so I don't pay interest.