Are you using multi-city to search? The ITA/Google Flights search code is broke because it doesn't check married segment inventory when quoting fares with multi-city search (it only checks inventory on the individual flight segments). Once you've pieced together the segments, it does a final check on the married segment inventory to see if there is really inventory there (and not just on the individual segments). If not, it will re-quote the correct fare for which there is actual married inventory available in the buckets. At least with DL flights it does this final check. With AA/UA flights, it does not seem to do this final check and will send you off to aa.com or united.com with a bogus fare quote. If you do a one-way search instead, all is well and it will correctly check the married segment inventory on the multi-segment flight options it quotes.