Old Gold:

Two things can happen to cause this. The first is much more likely.

(1) The system is designed to display the maximum number of seats available for the TOTAL routing input. So, in the example you posted, if 1923 has F0, even if 438 pulled as an independent flight shows Fx, where x>0, 438 will show F0 when paired with 1923. So, again using your example, the fact that 2678 by process of elimination had one F seat, caused 438 to display this seat when paired with 2678.

You should note that an exception is sometimes found in the case of illegal routings, e.g. if you have been ticketed on an illegal routing [which can be the case in a variety of situations these days - for example travelocity.com is infamous for allowing illegal routings to be ticketed] the CO computers may not behave as per the above.

(2) It is always possible, of course, that things simply changed in the split second between times you checked.