I needed to change a trip for my wife recently. Change flights on AA.com versus cancel and book a brand new itinerary versus calling the EXP desk to make the change yielded three wildly different results. The EXP desk ended up being the lowest by several hundred dollars. .Ignoring just plain bad programming, airfare searches are always subject to Heisenberg's Uncertainty Principle. That is, depending how you do it your search might affect the available inventory, causing an identical search to return a higher fare later.