I encountered it as random as it could get, had been flying with my brother and my Mom (all over 18, so no real kids), both my brother and me with Laptops, my Mom just with her huge carry-on backpack and cam+photobag, they gotten through Munich security within 2 minutes, shoes off, all through xrays and going on, I had been handsearched, had to unpack my carryon, turn on the notebook, empty my beautycase, been searched by a second person (it was her first day on the job, so add 10 extra minutes!)...thank god we were there with plenty of extra time!
In South Africa either my Mom or my Aunt once just couldnt understand what the officer wanted (both speak not much english) so they just let her pass!