I'm always a bit wary relying on what info most obd readers show,
i've seen too many that draw from the wrong pids,incorrectly read fault codes and other issues.
True miles(kms) are stored in the BCM and are displayed in the instrument,what the reader is showing could well be incorrect.
Are you saying the miles displayed in the instrument panel is possibly less than the miles the car has actually done or am i getting that wrong ?