I think I know what happened.
For example this search does not show Incrementing an integer field. It does, however, show http://elasticsearch-users.115913.n3.nabble.com/Incrementing-an-integer-field-td3726950.html.
Since I was looking explicitly for discuss.elastic.co I didn't notice that in fact it shows the mailinglist archive page, and I guess it considers the Discourse entry a duplicate?