What are "additional points" awarded for?
Solution 1:
Sometimes the game will "forget" where certain points came from, and award them under additional points.
For example, I just received 2820 additional points BUT my jet score said that I'd scored 0 that round, but I'd actually scored the exact amount that I received in my additional points. It only forgot for the after round tally, as when I checked my vehicle tab, it remembered my actual amount of jet experience (I'd levelled up).
So it seems the varying amounts may be due to for instance, it not remembering that you got a particular ribbon, or a particular service medal.
Here's a video explaining what I mean. It's pretty bad quality, but as you can see, my jet score shows up as 0 when it's tallying points, and I gain 2820 "additional points". Then later when I switch to my vehicles tab, jets has +2820 experience.
Also, as of Back to Karkand (and I'm assuming future expansions will do this too) any assignments you complete will award you 5000 "Additional Points".
Additionally, any XP earned during a double XP weekend will earn you the same amount of XP extra under Additional Points.
Solution 2:
Additional points count when you have 4 or 5 ribbons and the screen doesn't have time to list them out and if you get points for certain kills like roadkill and spotting can fall into that category too
Solution 3:
I managed to get a +2000 Additional Points at the end of a round yesterday. Im pretty sure this was because I got a service star (100 total kills with a particular weapon), this star wasn't included in the list of ribbons so it fits the 'Additional Points' quite well.
Solution 4:
You might be observing the results of the match in terms of teams. If your team wins in a game of Rush, there is a Rush Winner pin awarded to each person on the winning team, which gives additional points.
Battlefield is predominately objective-based, so they have point systems in place to make sure you're rewarded for following the purpose of the game mode.
Solution 5:
If you look at the round stats on Battlelog, there are awards given for "Top Jet", "Top Tank", "Top Engineer", etc. Could it be that these +200 points are given out for those?