Why do I sometimes see different fares on different websites?

Airfares – and available seats – change constantly. Fares are filed centrally and then disseminated many times a day through a hierarchy of subscribers who then disseminate it to their subscribers, and so forth. While this process is very quick, it isn’t instantaneous, thus one system may reflect the change very slightly before another. Inventory sell-out is another common reason. Ever get to the end of the reservations process and get a message that the fare is no longer available or has changed? If someone takes the last available space at that price it may take a few seconds or even a few minutes for the inventory update to reach all systems so what was available when you first looked at the list of available options may no longer be available when you complete the reservation.