Care packs and E drops.
According to maps\mp\killstreaks\_airdrop.gsc
The Care Package and Emergency Airdrop (mega) have a think process in which it picks one of the other killstreaks is a lottery raffle style (the game actually refers to it as "Mario Kart Style"), with some having more chances to be chosen.
ammo 17
uav 17
counter_uav 15
sentry 12
predator_missile 12
airdrop_precision_airstrike 11
airdrop_harrier_airstrike 7
helicopter 7
helicopter_flares 5 (this is the PaveLow)
stealth_airstrike 5
helicopter_minigun 3
ac130 3
emp 1
nuke 0
mega_ammo 12
mega_uav 12
mega_counter_uav 16
mega_sentry 16
mega_predator_missile 14
mega_precision_airstrike 10
mega_harrier_airstrike 5
mega_helicopter 5
mega_helicopter_flares 3
mega_stealth_airstrike 3
mega_helicopter_minigun 2
mega_ac130 2
mega_emp 0
mega_nuke 0
Maybe that will clear up a little of the confusion.
Subject: Package Chances. 
