Sometimes the game will "forget" where certain points came from, and award them under additional points.
For example, I just recieved 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 leveled 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.