An airfare checked twice within an hour can differ, and the usual explanation is not that the airline watched the first search. Seats are sold in tranches, and tranches run out.
Seats are sold from fare buckets
Each flight is divided into booking classes, each holding a limited number of seats at a set price. The system sells the cheapest available class first and moves upward as each empties.
A price rise between two searches therefore usually means the remaining seats in the lower bucket were sold, possibly to someone else entirely.
The seat is physically identical. What changed is which allocation it is being sold from, and that allocation is shared with every other channel selling the same flight.
Allocations are managed continuously
Revenue management systems adjust how many seats sit in each bucket based on how the flight is filling relative to its historical pattern.
A flight booking ahead of expectation will have cheap classes closed early, while one booking behind may have them reopened to stimulate demand.
This is why prices sometimes fall as departure approaches, which contradicts the common assumption that late booking always costs more.
Route economics set the pattern
Business-heavy routes fill late with less price-sensitive travellers, so airlines hold inventory back and prices climb steeply near departure.
Leisure routes depend on advance bookings and behave differently, with unsold seats late in the cycle sometimes offered cheaply rather than flown empty.
Knowing which kind of route is being flown predicts the price curve better than any general rule about how far ahead to book.
Search caching creates apparent inconsistency
Search sites cache results to remain responsive, so a displayed price may reflect availability from minutes earlier rather than the current state.
The airline's own system holds the authoritative answer, which is why a fare sometimes changes at the point of payment.
The mismatch is a caching artefact rather than a bait-and-switch, though it produces the same frustration.
What actually helps a traveller
Flexibility across dates matters more than timing a search, because different departures draw from entirely separate inventories.
Setting an alert rather than repeatedly searching removes the temptation to read normal bucket movement as a trend worth reacting to.
Once a fare is acceptable against the trip's value, booking it is reasonable, since the mechanism offers no reliable way to identify the lowest point in advance. The lowest price is only identifiable afterwards, by which time the seat has usually gone.