Ah yeah fair. The numbers I quoted are median and I don’t have the source. They are from a quick search of financial news. Your index numbers are probably a more sound comparison of overall house prices.
But even using the index numbers it isn’t hard to see that housing prices do in fact go both up and down.
They are not going down because of lack of inventory. Look at commercial, some properties had an 80% discount. But that requires that supply overwhelms demand. It’s not happening with regular housing.