* The price data isn't reliable. I see random sales for way under market price in my area, probably family sales. Tax evaluations should be reliable, but they are often years out of date.
* The input variables aren't reliable. This condo went for less because it has really high condo fees, but the condo fees aren't in the listing. This other house says it has lakefront AND beachfront AND water views. These two houses have "decks", one is a tiny front porch, the other is a huge thing. These two houses have the same lot size, but one is all grass and the other is straight down a hill.
* There isn't much data volume. You are trying to come up with an accurate price for every house in a neighborhood, but only a few percent sell each year. So the one or two houses that sell determine the prices for all the rest, but the data for those houses is unreliable due to the previous two factors.