Not exactly sure bg, but it appears the assessed value is based on what the county appraiser determines is fair market value, not sales price. Here’s something I found on the Travis County website:

If I buy a foreclosed property, and I bring in my closing statement, will the appraisal district’s market value be reduced to the purchase price?

If the foreclosed property was given an adequate exposure to the market, there was no attractive financing associated with it, the closing and repair costs were typical, and the sale is representative of its neighborhood, then it will. If the price it sold for is not consistent with the marketplace of a property with similar quality and condition, it will not.