The estimate is built on real sales from this archipelago. You see every sale, and what each one does to your price.
An old sale is adjusted to today's price level, and a recent sale still weighs more. No sale is thrown out, and none is taken as it is.
An agent opens this every day. A seller sees it once, at the tensest point of the sale. They don't need the same thing from it.
A sale's weight is set by distance, date and similarity, on the same grounds in every estimate. Gut feel doesn't move the number. The question ”what is this based on” always has the same answer: sales you can open.