I am guessing this is determined by rollingstock availability, or are there other factors involved?
Of course this
is timetabling, so lots of different factors have to be juggled into place at once. Striking a balance between reliability (recovery time) and rolling stock usage is one, what paths are possible in the overall timetable is another. The other end may be more important, as at Waterloo where platform dwell and paths are highly constrained.
In this case the two journeys are very similar in length, but the Waterloo trains do a cycle in 3:30 while the
FGW▸ ones take only 3:00. They alternate slower to Redhill with faster to Gatwick, which is an added constraint. The time at the other end is not actually any longer than at Reading, so it looks as if the crew break is not the most important factor, at least in this relatively short round trip.
If two trains per hour will have to go to Gatwick, to justify the cost of that shiny new platform 7, I wonder what they'll come up with as a pattern then. So far the effect of having P7, and putting Gatwick Express trains into P5-6, is that some FGW trains now dwell in P2 rather than using P3 and briefly lurking in a siding.