You have to set 'fairness' aside and understand the entire premise of this type of testing is being a black box. You are not supposed to understand, know or expect the distribution.
USADA is testing people based on data they have:
1) Remember when Jon Jones had his suspension reduced for giving 'substantial assistance' to USADA? That means he named other athletes on PEDS, or other coaches/people providing PEDS. USADA testing athletes includes information they have from third parties that they may be cheating.
2) Data on your previous test values. If you have within limits but 'suspicious' values, this is going to require additional testing. Some PEDS are notoriously hard to catch because they have a short half life. USADA doesn't hate money/resources, if you see someone like Jiri getting tested repeatedly, its likely because they believe he may be on a short acting substance based on values, or someone credible giving them information.
3) Frequency and value of your fight schedule. Fighting in multiple title fights or marquee headline fights? Expect to see USADA more than guys who fight infrequently. Then combine 1-2-3 together and understand someone like Costa could easily have a combination of all these items in play.