Proof:
Look up historical solds vs "on the market." (ask a realtor for this info, keep calling you will eventually find one who will give it to you) Then look at zillow or any other site that tracks historical home prices. You will see the supply leads price, usually by months/years not days or weeks.
How can this the be possible? 1. Realtors, on average, are wildly misinformed and much more interested in making a sale than doing market research.
2. It does not pay for large investors to buy individual properties so you do not have savvy investors correcting market errors. (this does not apply to commercial property)
Also keep in mind that on the "housing ladder" it's always easier for someone with a big property to down size than it is for someone to move up. Mid to low end housing will have a completely different market dynamic than 2 million dollar estates.