Business as usual, not surprising at all. I go over every invoice I receive at my business with a fine tooth comb. Out of a hundred or so invoices I approve for payment every month, at least 10% of them will be wrong. And guess what, the mistake is NEVER in my favor, never. There are a lot of companies out there that live by the "charge as much as you can, and if the customer complains too much, we will refund it, if they don't don't, it's free money for us". Literally, I mean this is how a lot of places do business. I spent countless hours on the phone every month arranging credits for over billing and over charging.