Your observation is correct and just to answer your query, as per my experience and knowledge, sometimes to some threads, depending upon their captions and underlying content, a small cash credit, to acknowledge the contribution and appreciate it, is given by the site. This is not a regular thing but sometimes during the month one can observe it. One more thing to note is that the post should not have grammatical and syntax errors and should have ample clarity in its composition. This is not a regular reward programme and sometimes we see a few of them while sometimes none of the posts deserve that. It is not the financial gain that is intended there, it is the token appreciation of the writing in that particular post and definitely encourages the writer.
Thoughts exchanged is knowledge gained.