Why are some flights so much cheaper than others on the same route?
Because airline pricing is not a single price with variations — it is a system of fare buckets, restrictions and distribution channels designed to charge different passengers different amounts for the same seat.
Fare buckets. Each cabin contains many booking classes, each with its own price, rules and inventory. The cheapest buckets are small and sell first. Two passengers sitting adjacent may have paid several times different amounts because one bought earlier, or into a different bucket.
Restrictions are how the fares are separated. Cheap fares typically carry: no changes or expensive changes, no refund, no seat selection, no bag, minimum stay requirements, and advance purchase conditions. These exist to make cheap fares unattractive to business travellers who need flexibility — which is the entire purpose.
Why identical-looking flights differ:
Routing. A connecting itinerary is frequently cheaper than a direct flight, because the airline is competing for traffic between two other cities and you are filling a seat it would otherwise sell at a lower yield.
Point of sale. The same flight can price differently depending on the country of origin of the booking and the currency, reflecting local competition.
Consolidator and bulk fares. Agents buying inventory in volume can sell below published fares under contractual restrictions, which is why some agency prices cannot be matched directly.
Codeshares. The same physical flight sold under a partner's code can carry a different fare.
Error fares — genuine pricing mistakes, occasionally honoured and frequently cancelled.
Two cautions. Hidden-city ticketing — booking a longer itinerary and leaving at the connection — breaches airline conditions, can result in the rest of the ticket being cancelled and loyalty accounts closed, and does not work with checked bags. And very cheap agency fares often come with poor support when something goes wrong.
Compare the total cost including bags and seats.