If appraisals are based on comps, how do prices rise over time?

[Kay March]

This is a technical question that arises from the painful experience of having an appraisal come in low. It happened two years ago when a VA appraiser, citing comps, said that the price my buyer had agreed to pay for my single family house was too high by $10,000. Today, two years later, that house is worth about $40,000 more than the price my buyer was allowed to pay. So can someone please explain how prices in general can rise over time if appraisals are based upon comps?