Almost certainly married segment logic. The flights individually may have I fare bucket availability when purchasing flights separately on each segment. However, there is separate seat inventory when purchasing the segments together on a single fare (this is the married segment inventory), and there can frequently be no I fare bucket inventory open on the married segments even when it's open on the individual flights. Unfortunately, when you search hop-by-hop using multi-city, the fare search engine is only looking at the individual flight bucket inventory and will quote through fares based on that. It's not until you go to purchase where it does a check on the actual married segment inventory for the fare and can discover that it does not match what's available on the individual flights. The message is misleading because the fare was not actually available at all on these particular flights. It's just that the fare search engine did not discover that fact until the final checkout phase when it actually checked the married segment inventory.