I think that is exactly the reason. My train in the mornings is a 10 (442) or 12 (377) carriage train and for a while if you were sat in the back four carriages (377) you would indeed be in 'no mans land' whilst we stopped at Balcombe.
Of late however, we do seem to use the full length of the platform, so maybe the drivers have been told to use the full length of the 'up' platform regardless of announcements?
I can't really say if the announcements have changed of late as I usually have my headphones on so don't listen to announcements (as long as everything appears to be running normally).
