Apart from the fact that with TSA there's no rhyme or reason?

You're absolutely right. I wrote about this in an earlier thread. The output from the sensor doesn't need to be turned into an image; it can be processed to just give an indication of whether anything unusual is found (a set of "bright" pixels against a background of "dark".) My guess is that different companies have built the two systems and chosen to implement different outputs. But it's only a guess.