A checked bag making a connection travels a completely different route from its owner. Understanding that route explains why bags miss flights their passengers make.
The tag is the instruction set
When a bag is checked, the printed tag carries a code identifying the flight, the destination and the bag itself. Everything that happens afterward is driven by reading that code.
Sorting machinery scans the tag as the bag passes and diverts it toward the correct chute. Where a scan fails, the bag is routed to a manual station for a person to read and reintroduce it.
Manual handling is slower by a wide margin, which is why a damaged or obscured tag is one of the most common reasons a bag falls behind its passenger.
Transfer is a separate journey through the airport
On arrival, transfer bags are separated from those terminating at the airport and fed into the sorting system rather than to the reclaim hall.
They then travel through belts and tunnels toward the departure pier for the onward flight, and at a large airport that distance can be considerable.
The system has a throughput limit, and when several wide-body arrivals land close together the queue forms exactly as it would at a security checkpoint.
The hold closes before the door does
Loading finishes ahead of passenger boarding completion because the load must be recorded and the balance calculated before departure.
A bag arriving after that point cannot simply be added, since the paperwork would need redoing and the aircraft would lose its departure sequence.
This is why a passenger can board comfortably while their bag, held up minutes earlier in sorting, misses the same flight.
Airline agreements govern whether transfer happens
Bags are checked through to a final destination only where the carriers involved have an arrangement to accept each other's baggage.
Without one, the passenger collects the bag, clears any required formalities and checks it again, and the connection time required grows accordingly.
Separate tickets bought independently generally fall into this second category even when both flights are on the same airline.
Recovery runs on the same infrastructure
A bag that misses its flight is rebooked onto the next service with space, which on a busy route may be hours later and on a thin route the following day.
Delivery onward from the destination airport is a separate courier operation, and its timing depends on distance and local coverage rather than on the airline.
Keeping essentials in a cabin bag is less about distrust than about acknowledging that the two journeys are genuinely independent.