Hi-Lo %

In reading �Professional BJ� by SW, The point is made that betting efficiency is 97% while playing efficiency is only 51%. It states: �All of these tables on deviating from basic strategy are of minor value.�
Does that mean when I have big bet out and thus vary from BS I only get the play right half the time? If so, why both with the variations from Basic Strategy?
[I�m a new counter and still working on my skills.]