For a 12-element series with `rolling(window=4).mean()`, the result will have NaN for the first 3 positions (indices 0, 1, 2) and non-NaN values for indices 3 through 11 — that is 9 non-NaN values, not 4. The first valid window covers indices 0–3, the second covers 1–4, and so on through indices 8–1