It would be easy to assume that he made a mistake, but if he did this on purpose (more likely) I suspect the reason is that the "439 miles" is supposed to be imagined, whereas the "39 km" is a measurement.
Suppose I made an example that if you were traveling at 1/3 the speed of light on Earth, you'd weigh X times your resting weight. If you are American you measure your weight in lbs, so if I quote it in kg, you just need to convert it anyway. Therefore even though I'd say you were traveling 1e8 m/s, I'd quote your supposed weight in lbs.
Suppose I made an example that if you were traveling at 1/3 the speed of light on Earth, you'd weigh X times your resting weight. If you are American you measure your weight in lbs, so if I quote it in kg, you just need to convert it anyway. Therefore even though I'd say you were traveling 1e8 m/s, I'd quote your supposed weight in lbs.