ITA Matrix and Google Flights (which is based on ITA Matrix) have issues with married segment logic and bucket availability when performing multi-city searches and will list fares for which no bucket availability exists. They seem to have fixed it with Google Flights (although they initially list the wrong fares and only correct it once you have selected all the flights). The problem is that the search only looks at bucket availability on individual flight segments when quoting fares, rather than correctly looking at the married segment bucket availability. There is no way to actually purchase the lower fare as the fare is only supposed to be sold if there is bucket availability in the married segments.

This is pretty trivial to demonstrate. For example, do a multi-city search for SEA-SLC-DEN for May 16th on Google Flights/ITA for Delta flights. ITA will show availability of a $72 E fare (VAUVC2BN) on flights 2954 and 2504. While there is V bucket available on the individual flights segments, there is only X bucket available on the married segments. Google Flights will correctly upfare to a $105 X-basis E fare after selecting the flights and doing a check on the married segment bucket availability (although it unhelpfully suggests the lower fare may be booked by a travel agent which is simply not true).