I think its because its a less transient area compared to others so it will take longer to reflect the value of the area due to a relatively low turnover. I recently had this issue with a remortgage which was motivated by reducing interest rate due to a higher LTV. The index used by the bank vs the valuation by the surveryor had a delta of 100k! I think the house is probably in the middle/top of the valuations. Once more people capitalise on the value and sell up then the reflection of average will be more true however the area will also lose its feel.