Like the clever man said, service intervals. Car is due a service at 54k so would be nice to purchase a car fresh from a service.
Not actually true. These cars need servicing every 9k or every 12 months so if someone has done more or less miles than 9k in a year at any point in the cars life it will throw it out.
Eg if someone only did 4k in the first year, the next service would at 13k or 12 months, whichever came first. I don't think you are likely to see many cars that have been serviced at exactly 9k for each interval leading to a service being due and completed on exactly 54k..........