请教：COUNTY的assessment confusing

I've offered a short sale townhouse for 268K, which county assessment is 284K. Another house in the same community is 240 spf (above grade) bigger than my wanted house, same age, which county assessment is only $277, it was sold at $295K end of last year. I looked at that house outside, it was better than the one I offered and has bigger/better deck, both houses have finished basements. How could this be? I know the county assessment is not a good measurement, but they are in the SAME community and in fact very close. Can anyone shed some light? Thank you!