Stage 3 of the mission re-computes the report on 'a random half' to gauge sampling noise — but this code slices the front of a SORTED list, so its 'half' is just the three smallest amounts. That's a biased sample, not a noisy one. Fix it to draw an actual random half with random.sample (the seed is already set).
The break is on line 6 — but read the whole snippet first.