So I noticed this earlier while playing as the EFEC on 360, during the seige (3rd battle) of washington DC.
When you kill a defending unit, it dies without ever being downed due to the "fight to the last man" rule. However, because of this none of the attackers units get experience for killing the enemy. I noticed this at first when after playing several games I wasn't getting much in the way of promotions.
I reset my battalion, and tried it again. After 3 games, all of my deployed units had 3 games played, 0 hostiles defeated. Given that I killed a multitude of units, this didn't make any sense. I tried playing other maps, and found that I got kill credits normally.
The Bottom Line:
Unit experience is tied to downing/"defeating" enemy units, so on seige mode when enemy units are never defeated, only killed, the attacker cannot get experience for his units through combat.
Is this on purpose?