Just to be clear, I was referring to the estimate of the property at a certain fixed point in time (my purchase date) changing over time. I do expect current market price to change for my home. I was just saying that these sites will estimate a value that differs and shifts, even estimates of periods during which actual market price is an available data point. March 2017 may look like a great investment when you checked in January 2018, but really bad when you check in October 2018.
Anyway, I agree with you that the market is just what people are willing to pay.
And yes, it's perfectly valid for market price to decrease over time if an area becomes less desirable.