You are claiming on an insurance policy that you haven't actually paid for yet, so they are taking the rest of what you owe them from your payout, this is quite normal.
You may have been paying monthly for years, but there is still an annual policy that will be renewed at some point each year.
Say your policy runs from 1st Dec - 30 Nov each year and costs £360 per year and this is divided into 12 monthly payments of £30. If you make a claim in September, you haven't paid Octobers or Novembers payment so they would deduct £60 from any payout.