Two long-haul trips on identical flights can cost very different amounts depending only on the return date. The difference comes from fare rules attached to the ticket as a whole.
A return ticket is one fare, not two
Airlines file fares as products with conditions, and a return journey is priced against a single fare basis rather than by adding two one-way prices together.
Those conditions can specify a minimum stay, a maximum stay, permitted days of travel and whether changes are allowed.
Because the conditions apply to the whole ticket, altering the return date can move the booking into a different fare entirely rather than adjusting the price slightly.
Stay requirements separate two kinds of traveller
Business travel is typically compressed into a working week and is relatively insensitive to price, while leisure travel spans weekends and is highly price-sensitive.
A rule requiring a Saturday night away, or a minimum number of nights, filters between the two without requiring the airline to ask why anyone is flying.
The traveller who can satisfy the condition receives the lower fare, and the one who cannot pays for the flexibility they actually need.
Inventory is managed separately on each leg
Seats are sold in booking classes, each with its own allocation, and the availability of a cheap class on the outbound does not guarantee it on the return.
A return ticket generally needs the same fare level available on both legs, so a single busy return date can push the entire booking upward.
Shifting the return by a day often restores availability, which is why flexible date searches produce such uneven price grids.
One-way pricing follows different logic
On many long-haul routes a one-way ticket costs close to a return, because the fare rules that produce the discount require a return journey to exist.
Low-cost carriers price each leg independently, which is why the arithmetic that works on short-haul does not transfer to intercontinental bookings.
Mixing carriers can therefore be cheaper on some routes and far more expensive on others, and the difference is structural rather than a matter of luck.
Seasonality is written into the fare
Fares carry seasonal validity, and the season is usually determined by the date of the outbound flight even when most of the trip falls in a different period.
Departing a day before a peak season begins can therefore price a whole trip at the lower level, which is a genuine rather than illusory saving.
Checking the day either side of an intended departure costs nothing and frequently reveals a boundary that the search results do not explain.