Appraisals using GRM for Income Value

[Carmen Pellegrino]

Ok need some help, recently posted I am getting slammed on a couple of appraisals and need to understand how this works. I get using a comparable sale but there is also an Income Value. For my example the appraisal lists 3 comps with a GRM average of 85(GRM is value divided by monthly rent) yet the appraiser used 54. This took the appraised value from $148,000 to $94,000. Where does this number come from?