If they alter the screen times based on the incoming queue's composition, then it may not be so "random." (ie. "Oh, there's a scruffy bearded guy in line; hurry up and shoo this clean-shaven fellow right on through.")
I do the opposite. I have to divest a lot of stuff to go through the screener. On the other side, I note how far through the current screening the staffer is, and either speed up or slow down my collection based on that (belts can be quite fiddly if you are determined!). Just make sure you never look directly at them to give the game away.
It's only failed the once, when one screener tagged me and let heaps of people go before nabbing me. Apart from that, it seems to work okay.
