My theory is that it's intentional; change the rules for security personnel so they don't end up in a routine, change them for travelers so... they also don't end up in a routine. If travelers with malicious intent are stressed because of the lack of predictability, they'll be extra stressed when they're up to no good.

Of course, this is baby/bathwater territory as only a fraction of a percentage of people are up to no good, and of those only a percentage is a danger to others.

As with many things, criminals are a step ahead and know this and know how to keep calm, be patient.