But most people - even criminals - tend to only do or say things that are potentially in their best interest. For example, a criminal in a case with very little evidence will say he didn't commit the crime (even if he did), because he might get acquitted and set free through saying this. Conversely, a criminal in a case where there is a cast-iron case against him will usually plead guilty, because this will generally lead to a lesser sentence.
In other words, people generally make choices based upon the optimum expected outcome to them of making each particular choice. In this particular instance, you're suggesting that Knox consciously chose to pretend not to remember about the first call to her mother (even though she actually did remember it) - a choice which could only have negative implications for her. This runs contrary to human nature - even for Amanda Knox!
