For a long time the death toll for the 1906 San Francisco earthquake was supposed to be 600. Now, it is listed at 2000. For a long time the death toll for the Civil War was said to be 600,000. Now it is considered that 750,000 is a more accurate number.
In 1860s, some young man walked out of town and that was that. Nobody in that town would ever know what became of him. If somebody fell into a river or got eaten by a bear, there would be no way of ever finding out about it. It's not that world anymore.