It's definitely mission 9 level 10 enemies that give you the experience. I've noticed that the yellow progression bar at the top of the HUD only appears when I kill level 10s and not when I kill lower levels.
I had killed about 5 level 10s for the first progression to appear, another 5 for the second progression and the same for the third, however I've managed to kill another 60+ level 10s since this and the progression bar hasn't moved, which has me concerned and I think it's glitched.