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There is certainly some guesswork involved, but from what I see, this only occurs when there are two or more pages from the same URL that rank similarly for the same term. How you get two pages ranking similarly for a phrase is up to you.
...occur when two pages from the same URL are shown next to each other in the SERPs. The second page is indented for clarity.
Not exactly. If you want nested listings on the first page (default ten results) in Google, then you'll have to get two pages ranking in the top 10.
If you have a page ranking, say, #12, and another #19, you'll get positions 12 and 13, with the second result indented, on the second page (default ten results).
In order to show you the most relevant results, we have omitted some entries very similar to the nnn already displayed. If you like, you can repeat the search with the omitted results included.
The other way of doing that is to add &filter=0 to Google's URL for the first SERP.
Compare, for example, the first SERP for
greenish widgets
and then add a &filter=0 to that URL.
You may be able to demonstrate that you already "qualify" for nested listings, but you need to SEO the site so that they appear without the &filter=0 assist :)