This behavior is consistent with with seats maps presented to both E/F and Checkmytrip about 4-7 days before a flight.
I have observed it many times; most recently was when I was travelling in a Qantas 763 (25 J seat).
These don't have a 23F seat in
WHY - making 24F a desirable seat.
Up until a week before the flight seat maps were showing as:
Code:
AB DEF JK
23 OO OO
24 OOO OO
25 OO OOO OO
26 OO OOO OO
...
And 4 days before:
AB DEF JK
23 ++ OO+ OO
24 ++ OOO OO
25 OO OOO OO
26 OO OOO OO
...
O=Seat (allocated or not), +=Unavailable seat