Travelocity and Expedia get fares that are not on airline web sites - similar to consolidator fares. I used a UA codeshare on Expedia for an AC flight. The TA said they could not access the Expedia fare. On ual.com they claim to beat any price for their flights and then there is a long footnote about how this does not include special fares. I've booked on Expedia.ca for that reason when I would have rather done it on AC. The worst thing is that you lose the fare entirely. They tend to exist for a day or two if they are really good. Only NW seems to have an absolute lowest fare on their web site but we're now in the wrong forum.