Part of the answer is found on usajobs.gov when you look up the duties of these inspectors, who have 2 roles, one of which is to ensure SOP agrue checkpoint and baggage, or to consider it another way, "test" TSOs.

Quote:
Responsible for reviewing and analyzing assessment reports and
testing results with special attention to identifying unusual trends
or action that appear to have potential for developing a security
problem.


Formulates recommendations or alternative courses of action to
be pursued.


Assists in the development of security mitigation measures for
transportation systems.


Evaluates the integration of the new technology, traditional
security measures, and human factors to assure operational
effectiveness and efficiency of the overall security system.


Establishes and maintains partnerships among public and private
stakeholders to accomplish objectives.


Coordinates with the Headquarters Program Office to develop and
implement Standard Operating Procedures, field guidance, initiatives,
and standards to ensure consistency.


If you read through this it details that part of what they are to do is seek out weeknesses within SOP.

An example of my personal experience: an inspector came through the checkpoint, claimed a medical reason they could not remove their shoes. Fine, hey get additional screenig. They did NOT alarm, so their hands and shoes are ETD. They pass. They received a certain type of patdown that only touches Specfic parts of the body. After, they said the TSO failed. A small object was taped to a part of the body that would not be touched according to SOP.

What happens when red teams and ASIs come through is after, whether you catch them or not, they analyise your peformance. They do this everytime. And they tell you what you did wrong, what you did correct.

In this case, nothig was done incorrectly, there was no corrective action the ASIs recmmended. The TSO was upset because they were told they failed, were decertified, and they did everything correctly, according to the inspector. The STSOs attempted to argue with the ASIs that the TSOs should not be decertified, as there was no way he patdown would hav found the item. The ASIs (they come in groups, never alone) said that was the point.

BTW, this paticular patdown has been changed a year or so ago. It covers more of the body. Part of the reason for this change was because red teams a d ASIs found there was a paticular hole in SOP, and that paticular test was to provide evidence of it.