Well, for one, any kills that you get with the random killstreaks in the boxes don't count towards the next killstreak reward. They still count as kills, just not towards the next killstreak. Also, anyone can take the boxes, even members of the other team. So you might just be giving the OTHER team good killstreaks and not yourself. However, I have noticed (as you can see in my video here:
) my sentry gun from a care package adds towards my killstreak. Im not sure exactly why that happened, or what affects your killstreak. Kind of odd.