Statement shows merchandise interest?? what is this?

I have just looked at my credit card statement after paying off the balance in full. on 19th apr it was £0.00. on 21st apr i was charged £13.97 merchandise interest (£1.527 a month). what does this mean? and can they do this? it is not the normal interest they charge.all that was on there was a balance transfer which was still under the interest free period, and i paid it all off, so the balance was nil!! how do they work out 13.97 on a balance of nil?help!
Answer
I am not sure but i think it will apply to my current free period.did you receive a credit offer up to a statement date? if you did when was the account cleared. for exampe free to statement dated 31/12/06. payment due date on statement say 21/01/07. you pay on the 15/01/07, however, credit free only to the 31/12/06(ie statement date). therefore mechandise interest charged at cards rate for nices for the period 01/01/7 to15/01/07.sorry late now, but hope it helps for the future.