I wonder if PhilPapers if optimized for Google Scholar crawling. The guidelines are at http://scholar.google.com/intl/en/scholar/inclusion.html#crawl . In my own case, versions of my papers that are archived on PhilPapers DO NOT show up as one of the versions on Google Scholar. Since PhilPapers is quite successful in getting many philosophers to upload no-paywall versions of their work, it would be great that it also shows up in other search engines.
In particular, without knowing all the technical details, I worry that some setup of PhilPapers makes it less accessible to indexing robots. Often, when I go from one entry to another, my browser history shows several distinct accesses. So I wonder if there are some subtle reloading happening at a level not visible to the end user but disruptive to bots.