Consider this like the free sample/taste that you get on your grocery store or membership club (Sam's, Costco, ...) You try it, you like it, you buy it. There is no publicly known data on how to determine who gets it or not, hence the randomness of its nature. Its just a "freebie" to get you hooked.

Personally, and this is my own opinion, with the state of affairs on the government, TSA, lack of funds, etc etc, they are trying to drive up the enrollment numbers and get more people to sign up for it. All you need to do is look at the news stories about 2 months or so ago regarding long TSA lines at the airport with waiting times of 2-3 hours just to get through security. More reason to sign up and participate on the program.