Why do some practitioners seem to use the same points but the results are different?

This is because better skilled acupuncture practitioners are using their mind to manipulate Qi and they use subtle hand techniques to manipulate and stimulate the needles in order to amplify their effect.  If you do not know what to watch for, you will not know that the practitioner may be doing something different than another. 

The same is also true of the choice of which acupuncture points to use.  One practitioner may use local points to treat a given set of symptoms, whereas another practitioner may choose a more strategic set of points which will treat the root of the problem rather than just treat the more obvious symptoms.