What the F*!*???!

My maths isn't, lets face it, brilliant. I can't do long division in my head and sometimes I have trouble with difficult things. But, for the most part, I can usually get fairly close to the answer in the countdown numbers connundrum thing on the back of the Guardian G2 section so I'm not that bad. I am, however, failing to see how on EARTH when putting £700 into an account that is, say, rather overdrawn (rather more than £700's overdrawn) I manage to be EVEN MORE OVERDRAWN than I was before the money went in. For me, and my bad maths, this does not WORK! How did they work out that putting in a few hundred pounds makes me over £100 more overdrawn than I was in the first place? Answers on a postcard please...