Assessed Values for Land In Goochland County

While researching sales of vacant land of parcels in the 30-75 acre range in Goochland County recently, I noticed that there have been several sales in recent months that have sold for less than the county's assessed value. All parcels of land are unique and different in some way and there is no locality that I am aware of that inspects every parcel of land in the county. Usually the county hires a firm that does "blanket" appraisals.( a generalization of value for land parcels usually of similar size within the county )Sometimes it seems a value is derived from one sale. Although I understand why it is not practical time -wise for these firms to do an in depth appraisal,it seems that they should at least recognize a trend that sale prices have leveled off or may have declined slightly from a couple of years ago. Oh well, maybe that will be reflected in next year's assessment