Differences in willingness to correct errors made between human and computer interactions
If I understand your question correctly, you have found that participants are more willing to admit to an error if they interact with a computer rather than a human being.

I would suggest reading up on related findings in the field of social desirability. Social desirability is the tendency of humans to think of themselves (self-deceptive enhancement) and try to present themselves towards others in a favorable way (impression management). There have been studies that suggest, that the effects of social desirability--or more precisely impression management--on reporting of sensitive or stigmatizing behavior are attenuated by interaction with computers rather than people (Turner et al., 1998; Joinson, 1999; King &amp; Miles, 1995; Davis, 1999; Reips, 2011). 

I suspect this effect is mostly a result of self-administration rather than being specific to computers, but may be a plausible explanation for your finding: Admitting mistakes is much more threatening to a person's self-worth than justifying a decision-- especially, when an experimenter is present.