Appraisal is an art, not a science. The large gap between the two appraisal opinions you received does not necessarily lead to the conclusion that mistakes were made. However, if you think that one or more of your appraisers did not do a proper job you should contact a lawyer for advice as to your legal remedies.
In my opinion one of the appraisers probably did not due their job. They should not be out that much. Read through the appraisals and determine which one is legitimate and which one isnt. Then report the poor report to the AIC.