Hello Captain Pat. I do a lot of valuations for banks (have done over 700 in the past 2 years) and I know that the differences in valuation that you describe are not unusual at all. Have you spoken to the appraiser at all about his/her appraisal? In your price range, $26,000 - $40,000 variation is not uncommon especially if the opinions were not all prepared at the same time. I venture to say that not all three professionals used the same comparable sales and listings as a basis for their evaluation. If your property has been on the market for nine weeks for $875,000 and you have not sold it, you can conclude that the appraised value was too high.
I would take a close look at all the comparable sales and see how closely they match the features of your property. The real estate market is an ever changing picture. There are times when I am asked to prepare a valuation for the same properties at different times (with 4 to 5 months in between) and my opinion is not always the same as I will have to rely on more recent sales data that was not available when I prepared the first evaluation. Sometimes, I will also have an appraiser call me in regards to a listing that is under contract as the appraiser would like to know how soon it will close so that he/she can use it as a comparable sale. Unless I received a full price offer, I cannot disclose the accepted price prior to the close of escrow. This means, there are sales data in the making that are not available to the appraiser when the appraisal is prepared and an appraised value has very limited shelf life.
As the other's have already pointed out. An appraisal is an opinion that is prepared by a licensed appraiser based on uniformly accepted principals and we all know that we don't always agree on everything. If you had two appraisals prepared at the same time by two different appraisers, there's a good chance that they would not have arrived at the same value.