It doesn't know if you've refitted the kitchen or built an extension or knocked down a bungalow and built a mansion. It doesn't know if the previous owner bought it off his dad cheap or overpaid massively because he's a div.
Ignore it. And when you're done ignoring it ignore it again.
How do you guys go about using zoopla? Is it worth looking at previous and estimated values?
Is an increase of 20k acceptable? If its 20k over the estimated, is that acceptable for London?