I would hazard a theory that such experiments are spread out so they won't attract too much attention as anything other than natural shifts, but that would not explain the current frequency of effects in here.

[They had been affected by a number of shifts as far back as before the last draft.

...]

But we know that Zompania was also affected by John's interference. Who is to say he did not manage to run personal experiments in that enclosure prior to the draft?

[The fact that his technology allowed him to reach the Lucetians and cause a shift of that magnitude was significant, even if they didn't know the specifics of how that shift had worked.]