USA Today reported that many home sales are not going through because the appraisals are too low to support the mortgage. At one point it reports complaints from realtors that appraisers now often come from outside the area and make low appraisals because they don't know the housing market.
If appraisers are unfamiliar with an area then it would be expected that they would make inaccurate appraisals. This would mean that there might be mortgages that don't go through because an appraisal comes in too low, however some mortgages may end up being issued that should not be because the appraisals are too high. There is no obvious reason that the appraisals would be biased on the low side.