Had a similar situation where the contract said it had 3,000 miles, but I returned it and it actually had 13,000 (I never even looked). Guy checking it in made me go to the counter, they laughed and said it wasn't possible to drive that much and they'd take care of it.
I wasn't worried, it's unlimited miles, but it is curious the system would let you go backwards in miles because of an obvious mis-key by someone.