Appraisals: Art or Science?

Last night I reviewed an appraisal that was just done for a client of mine that is refinancing their home and was surprised to note that the value of the square footage adjustments were way below the average used and therefore resulted in the home appraising for less than I believe the home is worth. As a real estate broker I am often asked to prepare a form known as a "BPO" or Broker Price Opinion for real estate attorneys who may be working on an estate and need to establish value. Banks who may be in the process of foreclosing on a mortgage will also ask for "BPO's" since it is less expensive ($75-$125) than ordering a full blown appraisal ($300-$500).
Anyway, according to my numbers this home should have easily appraised for at least $30,000 more than it did which leads me to my belief that real estate appraisals can often be more art than science.