Insurance is always out to ***** you

I Cancelled my Discovery Vitality at the end of November, expecting the last deduction to be for end of December. At the end of January another deduction went of andf when I queried it, I was told thta billing is in arrears (which I knew) and that January billing was for December. This makes no sense as my pay dat is the end of each month and if billing is in arrears then the end of December should have been my final deduction.