As I've mentioned, Derek, in over 20 years, we have cleaned to a very high standard, with absolutely no problems, however, we have often found residues from previous cleaning, including well known franchisees.
I would assume such people are trained to follow guidelines, religiously, but still fail to get " the best possible result "while others, gain invaluable experience and knowledge through working and experimenting.
I always advocate training, but have been on this earth too long, to expect people to become better people, simply through training.
Doug has just " hit the nail on the head "