Hey! Is that because you’ve added a conservatory since you bought?
You can see a little bit from one of the times I last spoke about the nerdier parts of valuations
One of the fun (read: challenging) things around valuations is that they’re only ever an estimate until you sell to someone else and it gets ‘locked in’, and maybe all companies (us included) should move away from the illusion of false accuracy! 
Right now, we try to extrapolate from purchase price and date in your local area for your property type up to present day, using Land Registry and Office for National Statistics data.
That being said, there’s some really cool companies that get super-granular on most of the properties in the UK (using things like satellite imaging, LIDAR, previous sales and lettings listings, planning applications) to build a richer view of a property and its neighbours. I actually brought one of those companies in to help our Insurance team set up the new buildings and contents insurance product! 