My husband has a 1997 Corolla in which the odometer spontaneously switched from registering miles to kilometers a few years ago, and he’s not the only Toyota owner it’s happened to. If your mile marker test shows that your odometer is actually off, that may be what happened.
My first car was a VW bug. I kept the speedometer/odometer cable disconnected most of the time. I reconnected it for state inspection and the owner of the garage gave me a :dubious: and asked me how come I only put 4 miles on in a year.