Unfortunately you never looked at your bill WITHOUT the insurance charge on it. You have 1 month from the date of purchase to add the insurance to your phone. This ALL could have been avoided if you would have simply looked at your bill.
If you ALWAYS get insurance, it would seem that your bill must have dropped since there was no longer a charge for insurance on it. I certainly would have CLOSELY looked at my bill if all of a sudden it dropped by $10/month immediately after a phone purchase. I would certainly have wondered why.
If you had also changed something else on your account which may have ALSO been a reason for a price drop, I would certainly have checked my bill to make sure I was being billed correctly for the account changes.
Amazing what problems you can have when you don't do something as simple as looking over your bill each month.