My suspicion on the TSA definition of risk, and risk based screening:
Low Risk: This person has been through the checkpoint 100 times in the last year. They know exactly what to do. They know what objects and materials attract attention and repeatedly successfully negotiate the gauntlet of the CP without additional scrutiny or the need to voluntarily surrender items. They provide little opportunity to catch things for which the TSA can take credit for saving aviation yet another day. They are a waste of their time and as such they are declared low risk so they can get them out of the way for the moderate risk people.
Moderate Risk: These folks fly fairly often, but not quite often enough to keep up with the myriad of rules and requirements that must be negotiated successfully for a lot of airports. They forget how early one must arrive and may be in a rush and occasionally forget and leave water bottles in their carry on. These folks provide a significant opportunity for the TSA to pad their numbers, but they are savvy enough and experienced enough to not bring really stupid things through the CP.
High Risk: What, fireworks are not allowed? How the heck do I blow up a plane with an M-80. Its just a screwdriver. Well, a big screwdriver, but screwdriver nonetheless. Hey, mama, you did remember to leave your Glock in the car didn't you? What do you mean I can't have a pocket knife? I feel naked without a pocketknife. You mean I have to pay $3 for water and can not bring my own? (In other words, the travel naive that make a lot of mistakes and add all sorts numbers to the statistical record.)
Highest Risk: Foreigners, old people in wheelchairs, anyone in diapers, nursing mothers, people who have injuries affecting mobility, people with implants, people with prostheses, and people taking pictures.