Upgrade at airport - availability but no seat
 

2-Hours before departure Expertflyer showed availability in various Business booking classes, including O-class. I wanted to use this seat to upgrade my U-fare back to AMS.
At the airport ticket office they told me I couldn't upgrade because the flight was full. On the monitor they showed me 29 seats booked, 30 seats taken.

Can some one explain why upgrade availability is shown when there is no seat available? J/C booking class I can understand but O-class not.

Maybe they dared to oversell?

Sure, but why make the award booking class available for overselling as well two hours before departure and then not have the seat? Doesn't OLCI book into Z ?

Now 1-hour before flight and Expertflyer shows following availability:
J1 C1 D1 I1 Z1 O1 Y9 B9 M9 U9 K9 H9 L9 Q9 T9 E9 N9 R9 V9 G9 X9

Overselling is exactly that - selling more seats than are available.

Just because everyone is checked in doesn't mean that everyone will turn up.

As for ExpertFlyer, perhaps it's accessing a cached/old version of the flight data, or maybe it's just wrong. If, at the airport, they couldn't upgrade you, perhaps that means they are seeing O as 0, despite you seeing O as 1.

Expertflyer is a 3rd party tool. The data is not guaranteed to be correct.

I don't think any tool shows "correct" availability once OLCI starts. At that point, only the folks at the airport knows what is going on and how many seats that are actually available on the plane.

ExpertFlyer does not used cached data. We retrieve the data from the airline reservation systems when you request it.

That is correct, once the flight goes under airport control the reservation system availability may not be updated by the airline in real time anymore, if at all.

EF may not be doing the caching, but that seems like a confirmation that the data accessed by EF may, in some cases, be old/cached/out of date.

Not necessarily. Don't forget, the airline reservation systems were built with the purpose of selling tickets, not disseminating load information. Once a flight goes to airport control, the airline probably doesn't want to sell tickets via travel agencies. So it would make sense that they don't update the availability information for the world to see, maybe even in purpose.