The mass appraisal system that most, if not all, counties use isn't perfect by far. In addition, the "appraisal" of your house may have been conducted 3 years ago (takes that long to appraise all of the property in the county).
If you're going to appeal your value to the board make sure to bring documentation of the property value. Just saying "it ain't werth dat much, me naibors house sold last month fer less" won't cut it. Neither will bringing in list prices from the newspaper or a real estate agents opinion. An actual independent appraisal is always best. But the easiest thing may be to look for inaccuracies in the informaiton. Square footage, quality (above average, etc.) number of bathrooms, anything that could throw it back in your favor.