Perhaps some re-ed-ucation is in order....
If you return a positive result, the machine is saying it thinks it smelt something interesting there...let me have another go.
If you then return a negative on the second test, a third test is done. If its also a negative, you're on your way.
If you return a positive on either the second or third test, then you will be subject to secondary screening...that is...a frisk search and a search of your carry on luggage...if you agree.
As the powers that be know, there are many innocuous causes of a positive reading. Therefore, the secondary screening is to ensure that you are not carrying "anything" that may have caused the positive return.
If nothing dastardly is found during secondary screening, it is assumed that your positive return was caused by one of those innocuous causes....and you are on your way.
Simple risk management practices to me......