TSA says you may get PreCheck via NEXUS or a separate registration in PreCheck but they also give it to random people based on undisclosed criteria on single occasions, and they also omit to give it to people with NEXUS (and presumably PreCheck registration) on single occasions. They don't want people to count on not being fully screened on the day of travel.

I was deeply disappointed not to get PreCheck leaving LAX given the gong show that it is in the regular line where AC leaves from (or left from if they've moved already). But I know that is one of the terms and conditions: don't count on it.

It would be nice if those who did get PreCheck without registration/NEXUS were at least frequent travellers that know how to follow a queue and put their crap on the belt in an organized way, and not take the darn shoes off if the sign says shoes stay on, but that is not always the case. NEXUS holders have been known to express dismay at infrequent travellers in the PreCheck lines -- sometimes in error, but sometimes because they won the boarding pass lottery and got PreCheck.

The signs on the PreCheck lines say something like "TODAY you have been selected to experience the PreCheck screening", to reinforce that the decision whether you get it (nominally) is not a guarantee either way.