In theory, that is correct. But 10 minutes of observation will reveal that it isn't quite as random as people passing a checkpoint. What happens is that once the person in the metal detector is just about finished being screened, the guards will begin looking for their next subject. At that time, 15-20 people are finishing up with their bag check, (or passing through the no bag line). Theses are the potential subjects. At this point it is mostly random. Most people do not manipulate the timing of when they pass through the bag check. (But it can be done. See below*). So who is among the 15-20 potential subjects is mostly random. But who among those 15-20 gets picked is not. The security person surveys those people and purposefully chooses which person s/he wants screened. I don't know what criteria they use, but they definitely pick someone out and head right to them. Some people just seem to get picked a lot. 10 out of 14 is a meaningful number as reported here. On our last trip, my brother was 8 out of 9. I was always entering at the same time and it was 0 out of 9 for me. 0 out of 9 for my daughter and 1 out of 9 for my wife. And if I am being honest about which of the four of us I would select if I were the security guard? It would be my brother.
*So how do you decrease your chances of getting picked if it means that much to you? Always go through the bag line. As the guard is checking your bag, scan around to watch as people get selected for extra screening. Don't leave the bag check table until someone in your area has just been picked. If you need to delay until someone else gets picked, just pretend to stumble and fumble with your stuff to buy some time. Fiddle with your zipper. Check your camera's lens cap. But be ready to move as soon as someone else gets picked and then walk through moving away from where the extra screening guard is. And if this doesn't work, then you weren't randomly selected. You were specifically chosen and there is nothing that you could have done.