As I often use GK drill for power training, because I have it on world class lvl and some drills not, I guess:
1) there is a random factor, as sometimes the outcome from single drills can differ. I.e. many times GK, then pass-go-shoot, can have an outcome like 25-20-23
2) Nik, your try with GK drills must have been after normal training? Seems like the efficiency falls with growing overall q. If I train a player from 50-60% on, there is a verx huge difference to the state where he is over 80%.