Nothing is random. Every word carries a strength score from 0 to 5. Miss one and it drops to
the front of the queue and comes back within a few rounds; get it right three times running and
it fades into the background so you stop wasting reps on words you own.
Mastered words resurface occasionally, just to check. Narrow the pool below and every
drill obeys it — pick Philosophy alone and Rapid Fire, Cloze, Match and Root Forge
all draw only from Philosophy.