I've noticed it too. I think they might have experimented with different programming and discovered there was an older-skewing audience for royal docs at that time.
It's probably because they know they can't compete ratings-wise with the likes of Britain's Got Talent, the Voice, etc., which will attract younger and bigger audiences, so are targeting viewers who don't want to watch these programmes.
They know they won't get massive viewing figures, which will reflect the relatively low budget and constant repackaging of the royal documentaries. They end up showing their 'better' documentaries on other days when they know they can get better ratings.
They probably think it's better to do this, than to commission high budget programmes to compete with the other channels, which would probably flop in the ratings.