House valuation

[BertiWooster]

http://www.nationwide.co.uk/hpi/calculator.asp ?calculate=true

On the screenscraper sites ( confused.com , Money supermarket etc ) - it asks for the value of your home and directs you to the above site , if you dont know .

I''ve put in the info for when i bought my home quite some years ago and it gives a particular figure .

However i've also put in data for a house a few doors from me , which sold a couple of years ago

Both houses are identical - yet the valuations between them are significantly different

How is that so - any ideas ?