Home appraisals are both factual and objective but there also is some subjectivity in the process. A professional and trained home appraiser will minimize any possible subjectivity, but some less trained or less professional appraisers may be more influenced by personal opinions. If you think the appraisal is incorrect or inaccurate, have a second appraiser come in and compare results.
Most home owners that have worked on their property and homes and really like their houses will over inflate the market value of the property. By studying the comps and doing some research home owners should be able to reasonably come to a fairly general idea of what their property is worth prior to the home appraisal.