An itinerary with a change of aircraft frequently costs less than the non-stop covering the same city pair. The pricing follows from what each seat is competing for, not from how far the aircraft travels.
A non-stop sells convenience
Passengers prefer to avoid a change, and business travellers in particular will pay a clear premium for it because their time has a cost attached.
An airline holding the only non-stop on a route is therefore selling a product its connecting competitors cannot match, and it prices accordingly.
The premium is largest where the demand is least flexible, which is why the same journey can look reasonable on a Saturday and expensive on a Tuesday morning.
Connections compete on price alone
A passenger considering a connection has already decided that the extra hours are acceptable, and is usually choosing on cost.
Airlines selling connecting itineraries know this, and they price to win the comparison rather than to reflect the additional flying involved.
Several carriers may offer connections through different hubs on the same city pair, so the competition is genuine and the fares track each other closely.
Hubs need feeding traffic
A hub airline builds banks of arrivals and departures so that passengers from many small cities can reach many destinations through one airport.
Those connecting passengers are what make a long-haul departure viable, since few routes could be filled from the hub city alone.
A cheap connecting fare is therefore not a loss. It fills seats that would otherwise fly empty and supports frequencies the local market could never sustain.
Each leg is priced from a different pool
Airlines allocate seats on a flight across many fare categories and adjust availability as a departure approaches.
A connecting itinerary draws from the cheapest categories still open on two flights, while a non-stop draws from a single flight where the popular categories may already be sold.
This is why the difference varies so sharply with timing, and why booking a few weeks earlier can invert the comparison entirely.
The saving carries real costs
A connection adds exposure to delay, since a missed transfer depends on the next available seat rather than on the length of the delay.
Whether the whole journey sits on one ticket matters more than the airline names on it, because separate tickets leave the passenger responsible for a missed connection.
Baggage handling, minimum transfer times and terminal changes at the connecting airport determine how much of the theoretical saving survives contact with a bad day.