Luck.
In the code, they all have an equal spawn chance:
actor BrutalFriend : RandomSpawner
{
DropItem "FriendMarine1"
DropItem "FriendMarine2"
DropItem "FriendMarine3"
DropItem "FriendMarine4"
DropItem "FriendMarine5"
DropItem "FriendDog"
DropItem "Sentrybot"
}