I think it makes more sense when you realise it was the replacement for Sixty Minutes (in turn the replacement for Nationwide), and the bulletins still had separate identities then so it probably felt alright to do that on the Six but not the Nine.
I don't think it was especially unusual, certainly at the time - you'll have seen on the Election 97 repeat the other day that they would regularly include a summary of quotes from the previous hour or so. Presumably it was considered a more elegant way of doing the closing headlines than just reading them all out again.
As you say, the Six was the replacement for Nationwide and Sixty Minutes, and so they did try and make it seem like more than just a bog-standard news bulletin, as you can see the billing for the first one goes on for ages -
https://genome.ch.bbc.co.uk/schedules/bbcone/london/1984-09-03#at-18.00
Up until then at teatime you had the news on its own, a straight bulletin, and then news analysis in Nationwide and Sixty Minutes, not only separate in the schedules but physically separate too with the news from the news department in TV Centre and the other bit from current affairs in Lime Grove. As The Last Days Of The Beeb by Michael Leapman points out, after Sixty Minutes had been a complete disaster, radical surgery was required, so we ended up with the Six but current affairs would still be making a contribution to it with short films and the like. So I think they particularly went out of their way to emphasise that the Six wouldn't just be a straight news bulletin for half an hour, partly because there'd previously been a bit of light and shade at teatime and they didn't want the audience to be switched off by just relentless news, and also partly to keep current affairs involved (though very much the junior partner after the failure of Sixty Minutes).
As Leapman points out, current affairs lobbied hard to keep Sixty Minutes going, not neccessarily because they thought it was any good, but because it was their biggest show. It's interesting to read about the failure of Sixty Minutes, one reason why it was launched in the first place was because they thought the combination of national news followed by regional news followed by news analysis with Nationwide was a weird one, and the two national bits should be together, but in the end Sixty Minutes had exactly the same schedule because the regions demanded to go at 6pm as it was the "best" slot, and putting the two national bits together would have meant it either starting much earlier or going on much later. So they ended up making all kinds of compromises to placate three different departments - news, current affairs and regions - so it looked a right mess. And after Sixty Minutes ended, the Beeb management were able to say it was all very well demanding to go on the "best" slot and news and current affairs having exactly equal airtime but the ratings were plummeting so radical change was required.
Anyone remember when ITV News relaunched in 1999, at the end of the Evening News they would have a montage/review of the day's events over the theme music.
Yes, although it was a bit of a waste of time - Trevor would always portentiously introduce it as "some of the images of the day", when in reality it was just a few disconnected clips from the various reports with the credits running over them. There were very rarely any really memorable or compelling images, so it was just a dull end credit sequence.
The Six O'Clock News in its first five years running for 35 minutes each night, then reduced to a simple half hour by 1989 - why did they chop off the last five minutes, which could be the "light hearted" bits of the programme?
They didn't, actually - for the first six months the Six lasted half an hour, with the regional news at 6.30 and the following programme beginning at the inconvenient time of 6.55 -
https://genome.ch.bbc.co.uk/schedules/bbcone/london/1984-09-03#at-18.00
When Wogan and 'stEnders began in February 1985, that stupid 6.55 junction went for a burton and the Six was extended to 6.35, with the regional news running up until seven. Presumably they didn't think the regions were able to fill an entire half hour (and I suppose in 1985 some of the regions would still be using film so they might indeed be struggling to get enough). It wasn't until October 1988, the same day the Nine was revamped, that the regional news lasted the full half hour and the Six went back down to thirty minutes.
Also by that point the current affairs department was being wound down and merged into news, so the requirement for a specific contribution by current affairs into the Six was no longer there.