I can't speak for the TSA, but here in Australia I am routinely "randomly selected" for bomb screening.

I have a terrible beard. I know it, my family knows it, my friends know it, people on who drive past on the bus know it. But it conceals the chinese phone directory beneath my jaw, so it stays.

But it definitely gets unkempt and quickly. If I forget to clean up before I fly, bam, I'm chosen for the bomb test. If I'm clean shaven or have trimmed that day, I sail through.

It's absolutely random, I'm assured. No bias whatsoever. When asked to show a random number generator the staff tend to get a bit grumpy. Appeals to the combinatorial unlikelihood of being chosen so many times in a row fall on dead ears.

Because my beard makes me look like a convert to Islam, I get bomb tested. It's as simple as that, in my opinion.