That sale price has shifted higher not just because of inherent home value but because of the mix of homes selling. Currently, more expensive homes are selling because there is a severe shortage of low-priced starter homes for sale. Higher volume on the pricier end shifts the median higher. If rental landlords suddenly dumped a few million low-priced homes on the market, and they sold fast, that median would shift lower immediately. Zillow's index is factoring in all homes, not just homes that recently sold, although that is the basis for comparison.

https://www.cnbc.com/2017/07/20/us-homes-are-now-more-valuable-than-ever.html