When the market changes, so does appraised value. For instance, if you were to increase the total market value of your property by building an addition, the appraised value would increase proportionately. Similarly, should your property's value be decreased by fire, the appraised value will decrease to reflect the downward effect on your property's market value. The entire community's economy and forces of supply and demand will also affect your property's appraised value. The PVA does not create the value.
People make their own property values by their transactions in the marketplace.
The PVA simply has the responsibility to study those transactions and appraise the property accordingly. This is the most common situation. 90,000 A property sells and the actual sale price is used to set the assessed value. Similar property, often located in the same neighborhood, has not sold in many years. The properties are equal, but the assessments are not.