Call Of Duty Modern Warfare 2 Killstreaks?

Hello, I play on the PC but I wonder why is it that whenever I get a killstreak, for example and harrier I get a harrier and I need 4 more kills for chopper gunner and the harrier gets 4 kills for me. But why don't I get chopper gunner? I always have to kill people with guns myself please help me if there is any settings.