The lowest fare on a flight search is rarely comparable to the one beside it. Basic fares are built by stripping out services and selling them back separately.
Why unbundling happened
Search engines and comparison sites rank results by price, and the fare shown first captures a large share of the traffic.
An airline that includes a bag in its fare appears more expensive than one that does not, even where the total cost of travelling is identical.
Competing in that environment requires the lowest possible headline number, which means moving everything optional out of the base fare.
What typically gets removed
Cabin baggage beyond a small item, checked bags, seat selection, changes, refunds and priority boarding are the standard exclusions.
Loyalty earning is often reduced or removed as well, along with eligibility for upgrades and, on some carriers, the ability to buy extras later at the same price.
Each of these is priced separately, and the prices frequently rise as departure approaches or when added at the airport rather than online.
Why ancillary revenue matters so much
Margins on the fare itself are thin and highly exposed to fuel prices and competition, whereas ancillary charges are far more stable.
These charges also arrive after the purchase decision, when the traveller has committed to the flight and the alternative is inconvenience.
For some carriers the additional charges account for a large share of profit, which is why the base fare can be priced so aggressively.
Comparing fares that are not comparable
The useful comparison is total cost including everything actually needed for the trip, which for most travellers means at least one bag.
Once bags and seats are added, a basic fare can exceed a fully inclusive one on another carrier, which the initial search results never show.
Time and connection quality belong in the comparison as well, since the cheapest fares often carry the least convenient routings and timings.
Where the restrictions bite hardest
Change and cancellation terms are the most consequential exclusion, because a non-changeable ticket has no value if plans move.
Bag size enforcement varies by carrier and is strictest where the fare is lowest, since the charge for an oversized item is a significant revenue line.
Reading what the fare includes before comparing prices is the step that makes the numbers mean the same thing across carriers.