Ranking #1 does not guarantee the featured snippet because Google chooses the snippet from whichever page provides the cleanest, most directly extractable answer, and that page is often not the top result. Position and snippet are two separate awards decided by two different judgments. One is about which page deserves the highest rank overall; the other is about which page hands Google a passage it can lift and display almost verbatim. A page can win the first and still lose the second.
The mechanism is extraction, not authority. When Google decides to show a snippet, it scans the candidate pages for a chunk of content that answers the query concisely and is formatted so it can be pulled out cleanly: a tight paragraph under a matching heading, a real ordered list, a genuine comparison table. The page that supplies that chunk wins the snippet even if it sits at position four or six. The #1 page may be authoritative and comprehensive yet bury its answer in three scrolling paragraphs, which makes it a poor extraction target. Google then reaches past it for a page that put the answer where it could grab it.
This is observed behavior worth keeping in mind rather than a fixed rule, because the snippet a query triggers can shift as Google reassesses which passage best fits. The practical consequence is that you do not earn the snippet by outranking everyone; you earn it by formatting an answer Google can extract without effort. Two competitors can sit one above the other, and the lower one walks away with the box on top of the results.
If a query you want shows a snippet held by a page below #1, treat that as your opening. Find the exact question being answered, then restructure your own coverage so the direct answer sits high on the page, under a heading that matches the query, in the format the box currently uses. Build for clean extraction rather than waiting for your rank to do the work, because rank alone will not hand you the snippet.