The cheapest fares often have combination restrictions (such as an A-B-A routing requirement or no side trips permitted). If that is true, they cannot be combined with other fares that don't meet these routing restrictions on a single ticket and the only way to book the fares is on separate tickets. For example, you can't nest the cheapest RDU-BOS fare within a cheap DTW-RDU fare on a single ticket as that would be considered a side trip which is prohibited. You can only book the fares on two separate tickets. The broken fare options in the fare listings are upfared to whatever fare that allows combining the fares on a single ticket. This is why when you want to take advantage of cheaper international fares out of JFK, you have to book the positioning flights from your home airport on a separate ticket.
Last edited by xliioper; Jun 3, 2019 at 3:03 pm