I suspect that the "least likely" are very often selected to make the process look random (statisticians will gag). The current random process translates to selecting the "most likely" (according to some arbitrary standard) and the "least likely". A 60 year old woman from Texas was strip-searched in AMS.

On the other hand, if the intent is to find unallowed items (performance measured by this), I suspect as a checker I would target the elderly and ignorant (which might translate foreign tourists in some uneducated eyes) as most likely to have not bothered to be completely "clean" thereby increasing the "success" rate of the search.
:-)