Why sometime historian consider Narrative of Slaves are bias and not true ?
Any writing is biased in therms of the author's viewpoint. The more emotionally involved the author is, the more likely that the account will be biased.
For example, the same behavior can be described with words that have the same denotation (dictionary meaning) but different connotations (emotional meanings). If you continue to do something inthe face of adversity, it can be described as "stubborn" or "perservering." The first term indicates disapproval, while the second term is more neutral or even positive.
Evaluative terms need to be supported by behavioral descriptions to clarify what has been done. "Good," "bad, "saintly,""evil" and other similar words are evaluative terms. Without support as to what makes things valued in either way indicates relatively greater bias.
I hope this helps. Thanks for asking.