The GatEx peak pattern is made more complicated by the Brighton extensions, so that some trains run through and others start from the sidings. I'm a little confused by the way they're currently shown to decamp to 1 and 2 in the PM peak, as you notice, though. Nothing is in 6 for that whole period, whilst 5 is being used as an extra down platform. We'll see if they change again before/at the actual time.
--- old post above --- --- new post below ---
I revise my opinion, I'm not confused - the 1 and 2 starters come up empty from Three Bridges on the slow, so it makes sense to start them there.