I am looking at a property worth $400K but the property tax assessment is $126K and Tax 9K.
How do they calculate the property tax assessment? It is obviously not the property value, then what is it?
There are multiple possible reasons why it is off:
Assessments are primarily about values relative to other properties. They need not have any connection with actual values. When it comes to figuring out the tax rate, the municipality adds up all the assessed values, divides the amount of money that they need by that sum, and the resulting percentage is everyone's tax rate. So if all of the assessments are in fact half of market value, changing them to reflect market value would double the sum of the assessments, and halve the tax rate, and you'd still pay exactly the same amount of tax.
In some places, the assessed value is a specific fraction of the market value. For example, in Downers Grove Township in Illinois, the fraction is one third. There a $126K assessed value would give a market value of $378K, consistent with your estimated market value of $400K.
To give more specific information, we'd need to know a more specific location. Not just country and state, but county and municipality.