Outside of this specific flight, I have noticed that UA.com can be very odd as to which flights it will show on a given search. The only discrenable pattern, as noted above, is that UA.com seems to steer away from expensive flights when there are cheaper options - for example, it doesn't seem to like to show J fares at all if they are all that is left.
For example, right now if I search for UA900 (SFO-FRA) on 10 September, using an economy search, it shows F5, A2, C4, J6. But if I search for a first class seat, UA.com won't show me that flight.