We flew to SFO on Jan 31 - only UD seats available at T-24 were 61 B & J. Not ideal being across the isle, but we thought it would be better than downstairs, so went for the UD seats.

On the return flight on Feb 8 there were a number of pairs of UD seats available at T-24, including 62J/K, which is of course the option we went for.

Know this doesn't answer your question, but does show (based on these two flights anyway), that it's all down to how busy each flight actually is.