$872 fare is a LNN4AQR1 fare which books into R. $623 fare is a GNX8AQI1 fare which books into I (as you noted). The fare basis codes can be found at end of booking process at fare rules link and can be looked up on ExpertFlyer. ExpertFlyer can also tell you what it sees for bucket availability on flights with Flight Availability search.




Looks like Google Flights may not be looking at married segment inventory (or perhaps it has stale cached inventory). ITA Matrix gets it correct with one-way search (showing R fare), but Google Flights does not. Generally, airline sites will do a final check for married segment inventory before allowing you to book (I know Delta's site does). They will dutifully pull up any published fare with API interface without checking inventory first. It's only when you checkout where they should do a final inventory check.