I realized after I posted this that account number is ambiguous in this context. It was my membership ID number at the credit union, not the bank account number

I realized after I posted this that account number is ambiguous in this context. It was my membership ID number at the credit union, not the bank account number itself. Their membership numbers were nine digits long and you would use them to identify yourself when you interacted with the a credit union representative over the phone. When I spoke to the support agent about that mystery car payment they said, “It looks like your member number is 649345121, and the real owner’s number is 649345122 so they must have mixed it up on the paperwork.” Which lead me to believe that the loan originated in house and the paperwork just asked for you membership number and which kind of account to use for the automatic payment. I guess the checksum was supposed to be the human who entered the number verifying that the account holder information matched the information on the form, but that didn’t happen in my case.