Two passengers in adjacent economy seats can have paid very different amounts for the same flight. The difference comes from how the cabin is divided for sale.
A cabin is sold as many products, not one
Airlines split each cabin into inventory classes, each with its own price and its own set of rules on changes, refunds, baggage and mileage earning.
Each class is allocated a number of seats, and when that allocation is sold the class closes and the next available one becomes the lowest price offered.
The seat itself is identical across several of these classes, so the price difference reflects the terms and the timing rather than the physical product.
Allocations are managed continuously
Revenue management systems forecast demand for each flight and adjust how many seats sit in each class as bookings arrive.
Stronger than expected demand causes cheaper classes to close early, while weak demand causes them to be reopened, which is why fares can fall as well as rise.
The forecast draws on the same flight's history, the day of week, the season and events at either end, so two flights an hour apart can be managed differently.
Business demand shapes the structure
Passengers traveling for work typically book late and value flexibility, and they will pay considerably more for a changeable ticket close to departure.
Leisure travelers book early and are price sensitive, so airlines sell them the cheapest classes far ahead in exchange for restrictive conditions.
The rules attached to each class exist to keep those groups from buying each other's fares, which is why cheap tickets carry advance purchase requirements and change penalties.
Availability is per direction and per segment
Inventory is managed for each flight segment separately, so a connecting itinerary needs the same class open on every leg to price at that level.
This produces the common situation where a one-way fare seems unrelated to the round trip, and where adding a connection can lower the total rather than raise it.
It also explains why a fare visible in a search disappears within minutes, since the class may have closed on one segment while the itinerary was being considered.
What the pattern means for booking
Because prices step between classes rather than drifting, small changes in date, routing or passenger count can cross a boundary and move the fare noticeably.
Booking several passengers together requires that many seats in one class, so a group can be quoted the price of the highest class needed to seat everyone.
Splitting a booking sometimes prices lower for that reason, at the cost of the seating and rebooking protection a single reservation provides.