70% comes up for a few reason.
- Texas law requires scores on licensure exams to be released to test takers. Normally if you fail you get a diagnostic with your score, but if you pass you don't get a score. In Texas everyone receives a score, even if they pass. But either NCEES or the Texas board applies an unknown curve to the raw score to normalize it to 70% as the passing score. Some people track this and use it to try and figure out the cut score. With enough data they could probably get pretty close, but it's a lot of effort for not a lot of useful info.
- I've seen more than one PE prep course tell students to aim for 70% as a should-be-realistic passing score.
But you're right. In reality, no one knows.