The mileage was probably recorded incorrectly when you checked the car out and/or back in. They may be able to figure it out based on their previous/next renter, but this is probably one of those situations where, unless you get a good customer service rep who is willing to do some digging, I fear it's going to be a tough row to hoe.

Follow-up and persistence are probably going to be key here, and in the end, it may come down to how much time and effort you want to invest given that your company will reimburse in any case. Just trying to be realistic and manage your expectations here.