The quote comes from the site, not the catalogue. Excavation volume, spoil removal, access, drainage and reinstatement are the variables, and a price built from a pool size alone is a guess that somebody eventually pays for.
Payments follow the build. Deposit, excavation complete, shell complete, plant installed, handover: staged against real milestones rather than dates, so neither side finances the other.
The trades run in order. Groundworks, shell, plant, tiling, fencing and landscaping each wait for the previous one, so a week lost at the start is a week lost everywhere and it has to be visible immediately.
Handover is where the reviews come from. Showing an owner how to run the plant, what to do weekly and who to call turns a finished pool into a satisfied customer rather than a confused one.