As Dr. House often says, people lie.
Another saw worth mentioning is that the victors write the histories.
While I wouldn't go as far as to call historical narratives unscientific quackery I would say that written accounts are the worst possible source of scientific evidence.
Let's say that we have a historical account which claims that Culture A was wiped out in the year 1550 by invading Marklars (just as a hypothetical).
We then find the cultural center of Culture A and start dating charcoal from fires and other artefacts. We find that there is absolutely no interruption of Culture A from the years 1300 to 1700. So what do we go with? The written account or the evidence acquired through modern techniques?
™ Version 4.2