I can only speak for Wisconsin, and only for a tiny portion of that, since it’s possible things are different elsewhere.
But here, the assessed value for tax purposes is intended to be a fair value based on the market; i.e., what an informed buyer would pay an informed seller at the time of the assessment.
The assessment figure is retained in the tax records unchanged from year to year unless a new assessment is done (the property changes hands, remodelled or razed, or a town-wide blanket assessment is ordered).
Every year, the county treasurer calculates the change in value (for one town/village/city at a time) based on the sales in that area. For example, if the total assessed value of properties sold was $1,000,000 and they sold for an aggregate $1,200,000, then the change was +20%. That number is used to multiply all other properties that have not sold to give the “Fair Market Value,” on the theory that the change in some properties affects all the others equally.
A ratio is computed from (total assessment in town) / (total FMV in town), expressed as a percentage. When that percentage gets too far from 100% for too many years, the state forces the town to revalue all properties and we start from scratch again.
If at any time, a property owner feels that his assessment is wrong, he may appeal, and bring any substantiating evidence (private appraisals and recent comparable sales) to the appeals board, which may adjust the assessment for the following year’s taxes. Obviously most people don’t appeal if their assessment is too low, but many appeal (and win) if it is too high.
To address the OP, it’s possible that the property owners are not as aware of the market as the assessors are. It’s also possible that the assessors made a mistake; there’s a lot of value judgement and opinion involved.