The ASSESSED value is determined by the County Assessor, and is determined by their formula and the County needs.
The MORTGAGE, (APPRAISAL) value is determined by just that, an APPRAISAL. This can be recent, or not. This can be based on the amount of the LOAN, just covering the Banks deriere.
The MARKET VALUE is really only determined when the property sells; that is, a ready and willing buyer steps up and puts their money where their mouth is. Truly, the only VALUE that is valid.