Sorry, I was in a hurry (lunch was waiting 
).
Okay, let's expand a bit.
Interesting what scripple found.
The URL I posted above, obviously refers to Elk 1.1 that has mainly 1 little difference: the use of some more html5 tags in probably more appropriate positions.
To be on the same line, I tested also an 1.0 site:
https://www.google.com/search?q=%22Adesso%20metto%20una%20seconda%20immagine,%20ma%20prima%20un%20po%27%20di%20testo%20per%20fare%20spazio%20e%20vedere%20quanto%20taglia.%22
and this, again, is interesting, even more because it's a direct (involuntary) comparison with an SMF 2.0 site.
So, what we see is:
1) the topic of the elk site is apparently not indexed,
2) instead the profile page is indexed,
3) the SMF site shows the topic.
Since scripple mentioned classes, let's see the chain we have in the three situations:
1) elk display page:
html
    body#chrome.action_messageindex.board_1
        div#wrapper.wrapper
            div#main_content_section
                div#forumposts.forumposts
                    form#quickModForm
                        div.post_wrapper.windowbg
                            div.postarea
                                div#msg4.inner
2) elk profile page:
html
    body#chrome.action_profile
        div#wrapper.wrapper
            div#main_content_section
                div#admin_content
                    div#profilecenter
                        div.windowbg.core_posts
                            div.inner
3) SMF 2.0 (PremiumMGC theme)
html
    body
        div#wrapper
            div#content_section
                div
                    div#main_content_section
                        table#sp_main
                            tbody
                                tr
                                    td#sp_center
                                        div#forumposts
                                            form#quickModForm
                                                div.windowbg
                                                    div.post_wrapper
                                                        div.postarea
                                                            div.post
                                                                div#msg_15026.inner
For the sake of comparison, elk 1.1 has:
html
    body#chrom.action_messageindex.board_3
        div#wrapper.wrapper
            section#main_content_section
                div#forumposts
                    main
                        form#quickModForm
                            article.post_wrapper.forumposts
                                div.postarea
                                    div#msg_28413.inner
What scripple is suggesting could make sense, the only two potential post-related identifiers in 1.0 are:
                        div.post_wrapper.windowbg
                            div.postarea
though, both contain the word "post", but is not the full name of the class, it's part of a bigger name.
So it could mean that google doesn't "split" the class name, but just takes everything and compares it with some list of words that may mean "content".