With regards to the values given by assessment tax records, it may work differently in other places, but here’s how it works in my county.
Each town, village or city (=municipality, think sub-county) is treated separately. Sometime in the past, the municipality is completely assessed and a dollar value is placed on each property. This is intended to represent the theoretical sale price at the time; it either reflects the actual sale, if recent, or a professional estimate of what that might be based on all available factors.
That number is the assessed value, and it never changes from one assessment to the next.
Since assessments are expensive, and the cost is bourne by the municipality, even tho it would be ideal to re-assess every year, that would be cost prohibitive, so reassessments are done only when forced by state law.
Every year, all of the actual property sales in the municipality are summed, and we now have two numbers; the total value of the sale prices and the total value of the assessment values for those same properties. Dividing one by the other gives a ratio of increase or decrease in values. This ratio is applied to all other, non-sold properties in the same municipality to give a “Fair Market Value” (FMV). So you see this figure will change for each property every year while the assessment value will not. The theory is that if property X sold for 127% of assessed value, property Y is now worth 127% more, too.
The total FMVs for one municipality divided by the total assessments gives us an assessment ratio; just after a new assessment, this ratio is 100%, but as time goes on, that number will change. When it gets too big or too small for too long a time (I don’t remember the state requirements in detail), the state forces a complete reassessment.
Now that you know how the two tax numbers (assessment and FMV) are created and what they mean, maybe it will help you interpret them. As Realtors, we take them as a guide but not gospel. And of course, YMMV – other counties may do it differently.