Okay, so how did you know the 4th ticket was not the last one in the V bucket? The bucket inventory is not inexhaustible. Also, booking the tickets could trigger RM systems to adjust the inventory.
Here's an example of stale cached inventory on Google flights. It thinks there's V availability on August 6 for 4221, 2305, 1151, but when you go to price it, it's no longer there and there is only X. ITA get it right and shows only X available.
My point is this - I was on the phone with the medallion support desk - while on phone, I was told by the agent that the fare on Google flights was no longer available. While
STILL on the phone with the agent, I booked 4 YYZ-HNL flights on Delta.com via Google flights. I booked them while on the phone to prove to the agent that the fare is obviously available. I even gave the agent my confirmation numbers, so she was able to see that they were in fact
CONFIRMED.
The agent then claimed that Google flights is not associated with Delta.com, even though I was directed back to Delta.com from Google flights.
There have been examples of Google flights showing wrong inventory. However, when trying to book on Delta.com, you get the error that the fare is no longer available. This time, however, the bookings confirmed.
My gripe is the fact that the agent is telling me that the fare is no longer available and the fact that I was
STILL able to book the fare after she told me this. Obviously the fare was available or I would not have been able to book it.