How it works is pretty basic:
Each airport terminal has a certain Pre line capacity, and it's known ahead of time (mostly) and a certain regular line capacity as well. They start putting passengers in the Pre line based on a risk assessment algorithm, taking into account age, sex, ticket type, number of passengers on your PNR or linked PNRs, and so on, with a random chance of it kicking you out to the regular line anyway. Having a Known Traveler ID number from the TSA, GE, or Nexus reduces your risk assessment significantly and puts you at the head of that line. Adding a KTID number at check-in can cause it to re-calculate, and there's a certain capacity flexibility to allow this to occur.
At some airports and times, the Pre capacity is so small relative to the number of TKID passengers that lots of people get dumped into the regular line, in others and at other times it's the reverse. At 5 AM at LGA's Central Terminal wing D, about 10% of all TKID passengers go through regular screening because of the heavy frequent traveler numbers, while at 11 AM almost 30% of non-TKID passengers get the Pre line.
The TSA published all of this a number of years ago in a congressional report, and it's remained the same.