A one-stop itinerary frequently costs less than the nonstop between the same two cities, sometimes by a wide margin. The reason lies in how airlines fill aircraft rather than in any difference in service.
Nonstop seats sell themselves
On a busy city pair the nonstop is what most travellers want, and it fills without discounting. Demand arrives unprompted, which removes any pressure to cut the fare.
An airline holding that inventory has little reason to sell it cheaply. Every seat released at a low price is one unavailable to someone who would have paid considerably more.
So the direct flight tends to be priced near what the route's least flexible passengers will bear. On corridors heavy with business traffic, that ceiling sits high.
Connections fill aircraft that would fly light
A connecting itinerary stitches together two flights that operate anyway, each serving its own local market. The connecting passenger is incremental traffic layered on top of that base.
If those aircraft would otherwise leave rows empty, a low fare that fills one is better than the alternative. The cost of carrying an additional person is small once the flight is committed.
That arithmetic lets an airline undercut a rival's nonstop while still improving the economics of both of its own departures. The discount is funded by spare capacity.
Hub geography decides how deep the discount goes
The cheapest connections are usually the least convenient ones, routed through a hub sitting well off the direct line. Those itineraries compete on price because they compete on nothing else.
A hub positioned neatly between origin and destination produces a genuinely competitive connection, so the airline does not need to discount it heavily to attract traffic.
The rough rule follows from that: the longer the detour and the longer the layover, the wider the gap between the connecting fare and the direct one.
The passenger pays in time and exposure
The saving is not free. A connection adds hours, a second boarding process and a period on the ground that has no value to the traveller at all.
It also adds a failure point. A delay on the first leg can consume the connection window, and recovery then depends on how many later flights the airline happens to operate.
Short layovers concentrate that risk, while generous ones spend the time you were trying to save. Neither choice removes the exposure completely.
Why the gap narrows near departure
Close to the day of travel the mix of buyers changes. Remaining demand skews towards people who must fly on a particular date and care comparatively little about price.
Airlines respond by withdrawing the cheapest fare classes across the board, connecting itineraries included. Those low buckets existed to capture price-sensitive travellers who book early.
What is left is a set of fares priced for urgency, where the connection still costs less but rarely by enough to justify the extra hours on the ground.