In John Levine wrote:

I have a NEXUS card which gives me a known traveller number. Usually
I get precheck on my boarding pass, but sometimes I don't, and I can't
figure out what the difference is. TSA vaguely claims random
something but I don't believe them.

It's supposed to random, to keep the bad guys from figuring out the system.
So they say.

On our last flight from MSP to ORF, before my wife and I got our numbers, I
"randomly" got "TSA PRECHECK" on my outbound boarding pass, and we both
"randomly" got it on our return flight.

The reduced hassle at security was enough to convince us to sign up.

--
St. Paul, MN