This is not strictly a penalty in and of itself, but it may be the result of one. Google stores its crawled
search data in two indexes: the primary index and the supplemental index. The supplemental index
stores pages that are less important to Google for whatever reason. Results from the supplemental
index typically appear at the end of the results for a Google query (unless the query is very specific),
and the results are marked as supplemental results.
Here is another great article about Google supplemental index you can look at.
