Here's what Autocheck said when asked about the discrepancy:
Thank you for contacting us.
Kindly note that Autocheck and Carfax are two different companies that different sources that report to each other. Since there are many ways in which an accident can easily go unreported, our Buyback Protection Program does not cover accidents for this reason.
We apologize for any inconveniences that this may have caused you and we appreciate your patience and understanding regarding this matter.
Customer Support: 888-409-2204, options # 1,1,5
Monday – Friday 8am – 7pm CST
Fax: (224) 698-5294
Experian Automotive Client Support