You might also note that the higher price did not violate DL's "Best Price Guarantee" as the fare was not higher. The logged in search simply failed to find the cheaper fare basis.

I used to use Matrix to verify I was seeing the lowest fares, now I just use google flights which has the same back-end (I think) but a much faster UI.

EDIT: Ruckzak, in the IT world, the difference between a bug and an "undocumented feature" is in the eye of the beholder. :-)