Your tax assessment is not your home value
Once a year your county sends an assessed value, the figure your property tax is based on. It is easy to read it as what the home is worth. It usually is not, and the gap can be large in either direction.
They are built for different jobs
A market estimate answers what a home would sell for today. An assessment answers what it should be taxed on this year. The second is deliberately smoothed, updated on a slow official cycle, and in many places capped in how fast it can rise, so it follows local tax rules rather than buyer behavior.
The gap runs both ways
In a fast-rising market, assessments trail and read low. In a flat or falling market, or in a state that caps how fast an assessment can rise, a home can be assessed above what it would actually sell for. Neither is a mistake. They answer different questions on different clocks.
Assessment ratios widen it further
Many places assess at a fraction of market value on purpose, using what is called an assessment ratio. A home might be assessed at 40 percent of its estimated market value by design. Reading that number as the home value understates it badly.
Which number to use for what
Property tax, or appealing your bill: the assessed value from the county.
A sense of what it would sell for: a market estimate, or an appraisal for the exact figure.
Tracking a trend over time: an index-based estimate like this one.