The market continues to change, sometimes daily.
So while market values change frequently, the assessment only changes when something has happened to the property that the assessor comes and looks at it. For Instance, you remodel or add on to your house. Maybe build a garage or pole building. Or perhaps buildings have been removed or destroyed. Another reason might be that the municipality orders a reassessment of all the property in its borders.
Assessments are only helpful to a very limited degree because they are a snapshot in time. they have some value only if you are familiar with the property or understand the market the property is located in.