The truth is that there are different types of appraisals, depending upon the purpose.
For insurance it could be replacement value
For a lender they will use comparable closed sales, they will want a conservative value
For market value, it depends which way the market is trending. If the market is going up, then looking at Active and Pending listings might be the best estimate
If the market is declining, then closed sales might overvalue the property.
The most common appraisal is for property tax assessment, and these are conducted on a periodic basis, depending upon the county. For example, in Los Angeles County, there are completed once every three years.
Just guessing at your true question, in most cases lenders want the loan value to be conservative. In a declining market it is common to have a re-appraisal prior to funding to verify that the market value has not declined to the point where the underwriter needs to have the loan re-worked.
Hope this helps, post again if needed.