It may also be because the likelihood of a substitution with a CIP 777-200 is greater, so once the aircraft operating the flight is confirmed to have F- which would likely be day of departure, upgrades may be processed. Just my theory, but as the 772 CIP program is in full swing and more frames are converted, chances are higher of having to downgrade confirmed F pax if sold/cleared in advance, which would create more disappointment and angry pax at the gate.