There is porbably a really obvious answer to this but my brain won't work round it
If for example your council is 1200 per 12 months but you pay for only 10 months of the year from april onwards. So actually pay £120 a month for those ten months.
Have you overpaid if for example you move in november.
(1200/10=120 1200/12=100 difference £20 a month. 7(months) X £20 = £140)
?