Market Value vs Assessed Improvement Value (taxable)

[Kenneth Bradford]

I went on the city government site and looked up a property with a market value at 91k, And the assessed improvement value (taxable) is at 81,800. This property has fire damage but the structure is sound. Is  the assessed improvement value based on a true # because the city inspected the property after the fire? Or is it an estimate based on the surrounding area. I'm in the Philadelphia market. Thanking you in advance!