Two passengers in adjacent seats commonly paid very different amounts. The cabin is not one product but a set of separately managed ones sharing the same physical space.
A cabin is divided into fare buckets
Each cabin contains numerous fare classes, identified by a letter, each with its own price and conditions. Seats are allocated between them rather than sold from a single pool.
Revenue systems open and close buckets continuously. When a cheap bucket sells out, the next available price becomes the lowest on offer even though seats remain.
This is why a fare can rise sharply with no visible change in demand. A single bucket closing moves the displayed price to the next tier.
Conditions differ as much as price
Cheaper classes carry restrictions on changes, refunds, baggage, seat selection and mileage accrual. The passenger paying more is buying flexibility as much as transport.
Advance purchase requirements and minimum stay rules attach to specific classes. Those rules exist to separate leisure travellers from business ones, who book late and cannot stay a weekend.
Separating those groups is the entire purpose of the structure. A single price would either lose the price-sensitive traveller or undercharge the time-sensitive one.
Route and origin change the fare filed
Fares are filed for markets rather than for flights, and a market is defined by origin and destination. The same flight carries passengers on entirely different fares depending on where their journey began.
A connecting itinerary can price below a direct flight on one of its own legs. Competition on the longer market is stronger than on the short one.
Point of sale matters as well. The same itinerary can be filed at different prices depending on the country the booking is made in.
Ancillary unbundling widened the gap
Basic fares now exclude items that were once standard, including checked baggage, seat assignment and changes. The headline price reflects a narrower product than it used to.
Adding those items back frequently brings a cheap fare above a mid-tier one. Comparing headline prices across airlines compares different bundles.
The unbundling is why fare comparison has become harder rather than easier. The relevant figure is the total for what the trip actually needs.
What this implies for booking
Prices do not follow a reliable weekly cycle, whatever folklore suggests. They follow the booking pace on that specific flight and date.
Flexibility across dates is worth more than timing a purchase. Adjacent days often sit in different buckets for reasons invisible from outside.
Watching a fare for a while establishes its normal range, which turns an unfamiliar number into a judgement. Without that range, any price looks arbitrary.