I’ve only missed one month after changing jobs – how am I in arrears?

i just started a new job. so i phoned csa to inform them i would be leaving my old job on the 27/09/13 and starting my new job on 30/09/13 and wanted to give them the information. but they told me i had to wait till i got my 1st wage slip from new job so i did.

now they took septembers payment from my old employer. octobers couldnt be taken as things werent sorted. now they have done my calculations at £174 pcm and say i am in arrears by £255 when i have only missed octobers

any help